Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

Introduction

For accessing data based on key lookup, the C++ standard library offers std::set, std::map, std::multiset and std::multimap. These are generally implemented using balanced binary trees so that lookup time has logarithmic complexity. That is generally okay, but in many cases a hash table can perform better, as accessing data has constant complexity, on average. The worst case complexity is linear, but that occurs rarely and with some care, can be avoided.

Also, the existing containers require a 'less than' comparison object to order their elements. For some data types this is impossible to implement or isn’t practical. In contrast, a hash table only needs an equality function and a hash function for the key.

With this in mind, unordered associative containers were added to the C++ standard. Boost.Unordered provides an implementation of the containers described in C++11, with some deviations from the standard in order to work with non-C++11 compilers and libraries.

unordered_set and unordered_multiset are defined in the header <boost/unordered/unordered_set.hpp>

namespace boost {
    template <
        class Key,
        class Hash = boost::hash<Key>,
        class Pred = std::equal_to<Key>,
        class Alloc = std::allocator<Key> >
    class unordered_set;

    template<
        class Key,
        class Hash = boost::hash<Key>,
        class Pred = std::equal_to<Key>,
        class Alloc = std::allocator<Key> >
    class unordered_multiset;
}

unordered_map and unordered_multimap are defined in the header <boost/unordered/unordered_map.hpp>

namespace boost {
    template <
        class Key, class Mapped,
        class Hash = boost::hash<Key>,
        class Pred = std::equal_to<Key>,
        class Alloc = std::allocator<std::pair<Key const, Mapped> > >
    class unordered_map;

    template<
        class Key, class Mapped,
        class Hash = boost::hash<Key>,
        class Pred = std::equal_to<Key>,
        class Alloc = std::allocator<std::pair<Key const, Mapped> > >
    class unordered_multimap;
}

These containers, and all other implementations of standard unordered associative containers, use an approach to its internal data structure design called closed addressing. Starting in Boost 1.81, Boost.Unordered also provides containers boost::unordered_flat_set and boost::unordered_flat_map, which use a different data structure strategy commonly known as open addressing and depart in a small number of ways from the standard so as to offer much better performance in exchange (more than 2 times faster in typical scenarios):

// #include <boost/unordered/unordered_flat_set.hpp>
//
// Note: no multiset version

namespace boost {
    template <
        class Key,
        class Hash = boost::hash<Key>,
        class Pred = std::equal_to<Key>,
        class Alloc = std::allocator<Key> >
    class unordered_flat_set;
}
// #include <boost/unordered/unordered_flat_map.hpp>
//
// Note: no multimap version

namespace boost {
    template <
        class Key, class Mapped,
        class Hash = boost::hash<Key>,
        class Pred = std::equal_to<Key>,
        class Alloc = std::allocator<std::pair<Key const, Mapped> > >
    class unordered_flat_map;
}

Starting in Boost 1.82, the containers boost::unordered_node_set and boost::unordered_node_map are introduced: they use open addressing like boost::unordered_flat_set and boost::unordered_flat_map, but internally store element nodes, like boost::unordered_set and boost::unordered_map, which provide stability of pointers and references to the elements:

// #include <boost/unordered/unordered_node_set.hpp>
//
// Note: no multiset version

namespace boost {
    template <
        class Key,
        class Hash = boost::hash<Key>,
        class Pred = std::equal_to<Key>,
        class Alloc = std::allocator<Key> >
    class unordered_node_set;
}
// #include <boost/unordered/unordered_node_map.hpp>
//
// Note: no multimap version

namespace boost {
    template <
        class Key, class Mapped,
        class Hash = boost::hash<Key>,
        class Pred = std::equal_to<Key>,
        class Alloc = std::allocator<std::pair<Key const, Mapped> > >
    class unordered_node_map;
}

These are all the containers provided by Boost.Unordered:

Table 1. Boost.Unordered containers

Node-based

Flat

Closed addressing

boost::unordered_set
boost::unordered_map
boost::unordered_multiset
boost::unordered_multimap

Open addressing

boost::unordered_node_set
boost::unordered_node_map

boost::unordered_flat_set
boost::unordered_flat_map

Closed-addressing containers are C++98-compatible. Open-addressing containers require a reasonably compliant C++11 compiler.

Boost.Unordered containers are used in a similar manner to the normal associative containers:

typedef boost::unordered_map<std::string, int> map;
map x;
x["one"] = 1;
x["two"] = 2;
x["three"] = 3;

assert(x.at("one") == 1);
assert(x.find("missing") == x.end());

But since the elements aren’t ordered, the output of:

for(const map::value_type& i: x) {
    std::cout<<i.first<<","<<i.second<<"\n";
}

can be in any order. For example, it might be:

two,2
one,1
three,3

To store an object in an unordered associative container requires both a key equality function and a hash function. The default function objects in the standard containers support a few basic types including integer types, floating point types, pointer types, and the standard strings. Since Boost.Unordered uses boost::hash it also supports some other types, including standard containers. To use any types not supported by these methods you have to extend Boost.Hash to support the type or use your own custom equality predicates and hash functions. See the Equality Predicates and Hash Functions section for more details.

There are other differences, which are listed in the Comparison with Associative Containers section.

The Data Structure

The containers are made up of a number of 'buckets', each of which can contain any number of elements. For example, the following diagram shows a boost::unordered_set with 7 buckets containing 5 elements, A, B, C, D and E (this is just for illustration, containers will typically have more buckets).

buckets

In order to decide which bucket to place an element in, the container applies the hash function, Hash, to the element’s key (for unordered_set and unordered_multiset the key is the whole element, but is referred to as the key so that the same terminology can be used for sets and maps). This returns a value of type std::size_t. std::size_t has a much greater range of values then the number of buckets, so the container applies another transformation to that value to choose a bucket to place the element in.

Retrieving the elements for a given key is simple. The same process is applied to the key to find the correct bucket. Then the key is compared with the elements in the bucket to find any elements that match (using the equality predicate Pred). If the hash function has worked well the elements will be evenly distributed amongst the buckets so only a small number of elements will need to be examined.

There is more information on hash functions and equality predicates in the next section.

You can see in the diagram that A & D have been placed in the same bucket. When looking for elements in this bucket up to 2 comparisons are made, making the search slower. This is known as a collision. To keep things fast we try to keep collisions to a minimum.

If instead of boost::unordered_set we had used boost::unordered_flat_set, the diagram would look as follows:

buckets oa

In open-addressing containers, buckets can hold at most one element; if a collision happens (like is the case of D in the example), the element uses some other available bucket in the vicinity of the original position. Given this simpler scenario, Boost.Unordered open-addressing containers offer a very limited API for accessing buckets.

Table 2. Methods for Accessing Buckets

All containers

Method

Description

size_type bucket_count() const

The number of buckets.

Closed-addressing containers only
boost::unordered_[multi]set, boost::unordered_[multi]map

Method

Description

size_type max_bucket_count() const

An upper bound on the number of buckets.

size_type bucket_size(size_type n) const

The number of elements in bucket n.

size_type bucket(key_type const& k) const

Returns the index of the bucket which would contain k.

local_iterator begin(size_type n)

Return begin and end iterators for bucket n.

local_iterator end(size_type n)

const_local_iterator begin(size_type n) const

const_local_iterator end(size_type n) const

const_local_iterator cbegin(size_type n) const

const_local_iterator cend(size_type n) const

Controlling the number of buckets

As more elements are added to an unordered associative container, the number of collisions will increase causing performance to degrade. To combat this the containers increase the bucket count as elements are inserted. You can also tell the container to change the bucket count (if required) by calling rehash.

The standard leaves a lot of freedom to the implementer to decide how the number of buckets is chosen, but it does make some requirements based on the container’s 'load factor', the number of elements divided by the number of buckets. Containers also have a 'maximum load factor' which they should try to keep the load factor below.

You can’t control the bucket count directly but there are two ways to influence it:

  • Specify the minimum number of buckets when constructing a container or when calling rehash.

  • Suggest a maximum load factor by calling max_load_factor.

max_load_factor doesn’t let you set the maximum load factor yourself, it just lets you give a hint. And even then, the standard doesn’t actually require the container to pay much attention to this value. The only time the load factor is required to be less than the maximum is following a call to rehash. But most implementations will try to keep the number of elements below the max load factor, and set the maximum load factor to be the same as or close to the hint - unless your hint is unreasonably small or large.

Table 3. Methods for Controlling Bucket Size

All containers

Method

Description

X(size_type n)

Construct an empty container with at least n buckets (X is the container type).

X(InputIterator i, InputIterator j, size_type n)

Construct an empty container with at least n buckets and insert elements from the range [i, j) (X is the container type).

float load_factor() const

The average number of elements per bucket.

float max_load_factor() const

Returns the current maximum load factor.

float max_load_factor(float z)

Changes the container’s maximum load factor, using z as a hint.
Open-addressing containers: this function does nothing: users are not allowed to change the maximum load factor.

void rehash(size_type n)

Changes the number of buckets so that there at least n buckets, and so that the load factor is less than the maximum load factor.

Open-addressing containers only
boost::unordered_flat_set, boost::unordered_flat_map
boost::unordered_node_set, boost::unordered_node_map

Method

Description

size_type max_load() const

Returns the maximum number of allowed elements in the container before rehash.

A note on max_load for open-addressing containers: the maximum load will be (max_load_factor() * bucket_count()) right after rehash or on container creation, but may slightly decrease when erasing elements in high-load situations. For instance, if we have a boost::unordered_flat_map with size() almost at max_load() level and then erase 1,000 elements, max_load() may decrease by around a few dozen elements. This is done internally by Boost.Unordered in order to keep its performance stable, and must be taken into account when planning for rehash-free insertions.

Iterator Invalidation

It is not specified how member functions other than rehash and reserve affect the bucket count, although insert can only invalidate iterators when the insertion causes the container’s load to be greater than the maximum allowed. For most implementations this means that insert will only change the number of buckets when this happens. Iterators can be invalidated by calls to insert, rehash and reserve.

As for pointers and references, they are never invalidated for node-based containers (boost::unordered_[multi]set, boost::unordered_[multi]map, boost::unordered_node_set, boost::unordered_node_map), but they will when rehashing occurs for boost::unordered_flat_set and boost::unordered_flat_map: this is because these containers store elements directly into their holding buckets, so when allocating a new bucket array the elements must be transferred by means of move construction.

In a similar manner to using reserve for vectors, it can be a good idea to call reserve before inserting a large number of elements. This will get the expensive rehashing out of the way and let you store iterators, safe in the knowledge that they won’t be invalidated. If you are inserting n elements into container x, you could first call:

x.reserve(n);
Note

reserve(n) reserves space for at least n elements, allocating enough buckets so as to not exceed the maximum load factor.

Because the maximum load factor is defined as the number of elements divided by the total number of available buckets, this function is logically equivalent to:

x.rehash(std::ceil(n / x.max_load_factor()))

See the reference for more details on the rehash function.

Fast Closed Addressing Implementation

Boost.Unordered sports one of the fastest implementations of closed addressing, also commonly known as separate chaining. An example figure representing the data structure is below:

bucket groups
Figure 1. A simple bucket group approach

An array of "buckets" is allocated and each bucket in turn points to its own individual linked list. This makes meeting the standard requirements of bucket iteration straight-forward. Unfortunately, iteration of the entire container is often times slow using this layout as each bucket must be examined for occupancy, yielding a time complexity of O(bucket_count() + size()) when the standard requires complexity to be O(size()).

Canonical standard implementations will wind up looking like the diagram below:

singly linked
Figure 2. The canonical standard approach

It’s worth noting that this approach is only used by libc++ and libstdc++; the MSVC Dinkumware implementation uses a different one. A more detailed analysis of the standard containers can be found here.

This unusually laid out data structure is chosen to make iteration of the entire container efficient by inter-connecting all of the nodes into a singly-linked list. One might also notice that buckets point to the node before the start of the bucket’s elements. This is done so that removing elements from the list can be done efficiently without introducing the need for a doubly-linked list. Unfortunately, this data structure introduces a guaranteed extra indirection. For example, to access the first element of a bucket, something like this must be done:

auto const idx = get_bucket_idx(hash_function(key));
node* p = buckets[idx]; // first load
node* n = p->next; // second load
if (n && is_in_bucket(n, idx)) {
  value_type const& v = *n; // third load
  // ...
}

With a simple bucket group layout, this is all that must be done:

auto const idx = get_bucket_idx(hash_function(key));
node* n = buckets[idx]; // first load
if (n) {
  value_type const& v = *n; // second load
  // ...
}

In practice, the extra indirection can have a dramatic performance impact to common operations such as insert, find and erase. But to keep iteration of the container fast, Boost.Unordered introduces a novel data structure, a "bucket group". A bucket group is a fixed-width view of a subsection of the buckets array. It contains a bitmask (a std::size_t) which it uses to track occupancy of buckets and contains two pointers so that it can form a doubly-linked list with non-empty groups. An example diagram is below:

fca
Figure 3. The new layout used by Boost

Thus container-wide iteration is turned into traversing the non-empty bucket groups (an operation with constant time complexity) which reduces the time complexity back to O(size()). In total, a bucket group is only 4 words in size and it views sizeof(std::size_t) * CHAR_BIT buckets meaning that for all common implementations, there’s only 4 bits of space overhead per bucket introduced by the bucket groups.

A more detailed description of Boost.Unordered’s closed-addressing implementation is given in an external article. For more information on implementation rationale, read the corresponding section.

Open Addressing Implementation

The diagram shows the basic internal layout of boost::unordered_flat_map/unordered_node_map and boost:unordered_flat_set/unordered_node_set.

foa
Figure 4. Open-addressing layout used by Boost.Unordered.

As with all open-addressing containers, elements (or pointers to the element nodes in the case of boost::unordered_node_map and boost::unordered_node_set) are stored directly in the bucket array. This array is logically divided into 2n groups of 15 elements each. In addition to the bucket array, there is an associated metadata array with 2n 16-byte words.

foa metadata
Figure 5. Breakdown of a metadata word.

A metadata word is divided into 15 hi bytes (one for each associated bucket), and an overflow byte (ofw in the diagram). The value of hi is:

  • 0 if the corresponding bucket is empty.

  • 1 to encode a special empty bucket called a sentinel, which is used internally to stop iteration when the container has been fully traversed.

  • If the bucket is occupied, a reduced hash value obtained from the hash value of the element.

When looking for an element with hash value h, SIMD technologies such as SSE2 and Neon allow us to very quickly inspect the full metadata word and look for the reduced value of h among all the 15 buckets with just a handful of CPU instructions: non-matching buckets can be readily discarded, and those whose reduced hash value matches need be inspected via full comparison with the corresponding element. If the looked-for element is not present, the overflow byte is inspected:

  • If the bit in the position h mod 8 is zero, lookup terminates (and the element is not present).

  • If the bit is set to 1 (the group has been overflowed), further groups are checked using quadratic probing, and the process is repeated.

Insertion is algorithmically similar: empty buckets are located using SIMD, and when going past a full group its corresponding overflow bit is set to 1.

In architectures without SIMD support, the logical layout stays the same, but the metadata word is codified using a technique we call bit interleaving: this layout allows us to emulate SIMD with reasonably good performance using only standard arithmetic and logical operations.

foa metadata interleaving
Figure 6. Bit-interleaved metadata word.

A more detailed description of Boost.Unordered’s open-addressing implementation is given in an external article. For more information on implementation rationale, read the corresponding section.

Equality Predicates and Hash Functions

While the associative containers use an ordering relation to specify how the elements are stored, the unordered associative containers use an equality predicate and a hash function. For example, boost::unordered_map is declared as:

template <
    class Key, class Mapped,
    class Hash = boost::hash<Key>,
    class Pred = std::equal_to<Key>,
    class Alloc = std::allocator<std::pair<Key const, Mapped> > >
class unordered_map;

The hash function comes first as you might want to change the hash function but not the equality predicate. For example, if you wanted to use the FNV-1 hash you could write:

boost::unordered_map<std::string, int, hash::fnv_1>
    dictionary;

There is an implementation of FNV-1 in the examples directory.

If you wish to use a different equality function, you will also need to use a matching hash function. For example, to implement a case insensitive dictionary you need to define a case insensitive equality predicate and hash function:

struct iequal_to
{
    bool operator()(std::string const& x,
        std::string const& y) const
    {
        return boost::algorithm::iequals(x, y, std::locale());
    }
};

struct ihash
{
    std::size_t operator()(std::string const& x) const
    {
        std::size_t seed = 0;
        std::locale locale;

        for(std::string::const_iterator it = x.begin();
            it != x.end(); ++it)
        {
            boost::hash_combine(seed, std::toupper(*it, locale));
        }

        return seed;
    }
};

Which you can then use in a case insensitive dictionary:

boost::unordered_map<std::string, int, ihash, iequal_to>
    idictionary;

This is a simplified version of the example at /libs/unordered/examples/case_insensitive.hpp which supports other locales and string types.

Caution
 Be careful when using the equality (==) operator with custom equality predicates, especially if you’re using a function pointer. If you compare two containers with different equality predicates then the result is undefined. For most stateless function objects this is impossible - since you can only compare objects with the same equality predicate you know the equality predicates must be equal. But if you’re using function pointers or a stateful equality predicate (e.g. boost::function) then you can get into trouble.

Custom Types

Similarly, a custom hash function can be used for custom types:

struct point {
    int x;
    int y;
};

bool operator==(point const& p1, point const& p2)
{
    return p1.x == p2.x && p1.y == p2.y;
}

struct point_hash
{
    std::size_t operator()(point const& p) const
    {
        std::size_t seed = 0;
        boost::hash_combine(seed, p.x);
        boost::hash_combine(seed, p.y);
        return seed;
    }
};

boost::unordered_multiset<point, point_hash> points;

Since the default hash function is Boost.Hash, we can extend it to support the type so that the hash function doesn’t need to be explicitly given:

struct point {
    int x;
    int y;
};

bool operator==(point const& p1, point const& p2)
{
    return p1.x == p2.x && p1.y == p2.y;
}

std::size_t hash_value(point const& p) {
    std::size_t seed = 0;
    boost::hash_combine(seed, p.x);
    boost::hash_combine(seed, p.y);
    return seed;
}

// Now the default function objects work.
boost::unordered_multiset<point> points;

See the Boost.Hash documentation for more detail on how to do this. Remember that it relies on extensions to the standard - so it won’t work for other implementations of the unordered associative containers, you’ll need to explicitly use Boost.Hash.

Table 4 Methods for accessing the hash and equality functions
Method Description

hasher hash_function() const

Returns the container’s hash function.

key_equal key_eq() const

Returns the container’s key equality function..

Comparison with Associative Containers

Table 5 Interface differences
Associative Containers Unordered Associative Containers

Parameterized by an ordering relation Compare

Parameterized by a function object Hash and an equivalence relation Pred

Keys can be compared using key_compare which is accessed by member function key_comp(), values can be compared using value_compare which is accessed by member function value_comp().

Keys can be hashed using hasher which is accessed by member function hash_function(), and checked for equality using key_equal which is accessed by member function key_eq(). There is no function object for compared or hashing values.

Constructors have optional extra parameters for the comparison object.

Constructors have optional extra parameters for the initial minimum number of buckets, a hash function and an equality object.

Keys k1, k2 are considered equivalent if !Compare(k1, k2) && !Compare(k2, k1).

Keys k1, k2 are considered equivalent if Pred(k1, k2)

Member function lower_bound(k) and upper_bound(k)

No equivalent. Since the elements aren’t ordered lower_bound and upper_bound would be meaningless.

equal_range(k) returns an empty range at the position that k would be inserted if k isn’t present in the container.

equal_range(k) returns a range at the end of the container if k isn’t present in the container. It can’t return a positioned range as k could be inserted into multiple place.
Closed-addressing containers: To find out the bucket that k would be inserted into use bucket(k). But remember that an insert can cause the container to rehash - meaning that the element can be inserted into a different bucket.

iterator, const_iterator are of the bidirectional category.

iterator, const_iterator are of at least the forward category.

Iterators, pointers and references to the container’s elements are never invalidated.

Iterators can be invalidated by calls to insert or rehash.
Node-based containers: Pointers and references to the container’s elements are never invalidated.
Flat containers: Pointers and references to the container’s elements are invalidated when rehashing occurs.

Iterators iterate through the container in the order defined by the comparison object.

Iterators iterate through the container in an arbitrary order, that can change as elements are inserted, although equivalent elements are always adjacent.

No equivalent

Closed-addressing containers: Local iterators can be used to iterate through individual buckets. (The order of local iterators and iterators aren’t required to have any correspondence.)

Can be compared using the ==, !=, <, <=, >, >= operators.

Can be compared using the == and != operators.

When inserting with a hint, implementations are permitted to ignore the hint.
Table 6 Complexity Guarantees
Operation Associative Containers Unordered Associative Containers

Construction of empty container

constant

O(n) where n is the minimum number of buckets.

Construction of container from a range of N elements

O(N log N), O(N) if the range is sorted with value_comp()

Average case O(N), worst case O(N2)

Insert a single element

logarithmic

Average case constant, worst case linear

Insert a single element with a hint

Amortized constant if t elements inserted right after hint, logarithmic otherwise

Average case constant, worst case linear (ie. the same as a normal insert).

Inserting a range of N elements

N log(size() + N)

Average case O(N), worst case O(N * size())

Erase by key, k

O(log(size()) + count(k))

Average case: O(count(k)), Worst case: O(size())

Erase a single element by iterator

Amortized constant

Average case: O(1), Worst case: O(size())

Erase a range of N elements

O(log(size()) + N)

Average case: O(N), Worst case: O(size())

Clearing the container

O(size())

O(size())

Find

logarithmic

Average case: O(1), Worst case: O(size())

Count

O(log(size()) + count(k))

Average case: O(1), Worst case: O(size())

equal_range(k)

logarithmic

Average case: O(count(k)), Worst case: O(size())

lower_bound,upper_bound

logarithmic

n/a

Standard Compliance

Closed-addressing containers

unordered_[multi]set and unordered_[multi]map are intended to provide a conformant implementation of the C++20 standard that will work with C++98 upwards. This wide compatibility does mean some compromises have to be made. With a compiler and library that fully support C++11, the differences should be minor.

Move emulation

Support for move semantics is implemented using Boost.Move. If rvalue references are available it will use them, but if not it uses a close, but imperfect emulation. On such compilers:

  • Non-copyable objects can be stored in the containers. They can be constructed in place using emplace, or if they support Boost.Move, moved into place.

  • The containers themselves are not movable.

  • Argument forwarding is not perfect.

Use of allocators

C++11 introduced a new allocator system. It’s backwards compatible due to the lax requirements for allocators in the old standard, but might need some changes for allocators which worked with the old versions of the unordered containers. It uses a traits class, allocator_traits to handle the allocator adding extra functionality, and making some methods and types optional. During development a stable release of allocator_traits wasn’t available so an internal partial implementation is always used in this version. Hopefully a future version will use the standard implementation where available.

The member functions construct, destroy and max_size are now optional, if they’re not available a fallback is used. A full implementation of allocator_traits requires sophisticated member function detection so that the fallback is used whenever the member function call is not well formed. This requires support for SFINAE expressions, which are available on GCC from version 4.4 and Clang.

On other compilers, there’s just a test to see if the allocator has a member, but no check that it can be called. So rather than using a fallback there will just be a compile error.

propagate_on_container_copy_assignment, propagate_on_container_move_assignment, propagate_on_container_swap and select_on_container_copy_construction are also supported. Due to imperfect move emulation, some assignments might check propagate_on_container_copy_assignment on some compilers and propagate_on_container_move_assignment on others.

Construction/Destruction using allocators

The following support is required for full use of C++11 style construction/destruction:

  • Variadic templates.

  • Piecewise construction of std::pair.

  • Either std::allocator_traits or expression SFINAE.

This is detected using Boost.Config. The macro BOOST_UNORDERED_CXX11_CONSTRUCTION will be set to 1 if it is found, or 0 otherwise.

When this is the case allocator_traits::construct and allocator_traits::destroy will always be used, apart from when piecewise constructing a std::pair using boost::tuple (see below), but that should be easily avoided.

When support is not available allocator_traits::construct and allocator_traits::destroy are never called.

Pointer Traits

pointer_traits aren’t used. Instead, pointer types are obtained from rebound allocators, this can cause problems if the allocator can’t be used with incomplete types. If const_pointer is not defined in the allocator, boost::pointer_to_other<pointer, const value_type>::type is used to obtain a const pointer.

Pairs

Since the containers use std::pair they’re limited to the version from the current standard library. But since C++11 std::pair's piecewise_construct based constructor is very useful, emplace emulates it with a piecewise_construct in the boost::unordered namespace. So for example, the following will work:

boost::unordered_multimap<std::string, std::complex> x;

x.emplace(
    boost::unordered::piecewise_construct,
    boost::make_tuple("key"), boost::make_tuple(1, 2));

Older drafts of the standard also supported variadic constructors for std::pair, where the first argument would be used for the first part of the pair, and the remaining for the second part.

Miscellaneous

When swapping, Pred and Hash are not currently swapped by calling swap, their copy constructors are used. As a consequence when swapping an exception may be thrown from their copy constructor.

Variadic constructor arguments for emplace are only used when both rvalue references and variadic template parameters are available. Otherwise emplace can only take up to 10 constructors arguments.

Open-addressing containers

The C++ standard does not currently provide any open-addressing container specification to adhere to, so boost::unordered_flat_set/unordered_node_set and boost::unordered_flat_map/unordered_node_map take inspiration from std::unordered_set and std::unordered_map, respectively, and depart from their interface where convenient or as dictated by their internal data structure, which is radically different from that imposed by the standard (closed addressing).

Open-addressing containers provided by Boost.Unordered only work with reasonably compliant C++11 (or later) compilers. Language-level features such as move semantics and variadic template parameters are then not emulated. The containers are fully AllocatorAware.

The main differences with C++ unordered associative containers are:

  • In general:

    • begin() is not constant-time.

    • erase(iterator) returns void instead of an iterator to the following element.

    • There is no API for bucket handling (except bucket_count).

    • The maximum load factor of the container is managed internally and can’t be set by the user. The maximum load, exposed through the public function max_load, may decrease on erasure under high-load conditions.

  • Flat containers (boost::unordered_flat_set and boost::unordered_flat_map):

    • value_type must be move-constructible.

    • Pointer stability is not kept under rehashing.

    • There is no API for node extraction/insertion.

Benchmarks

boost::unordered_[multi]set

All benchmarks were created using unordered_set<unsigned int> (non-duplicate) and unordered_multiset<unsigned int> (duplicate). The source code can be found here.

The insertion benchmarks insert n random values, where n is between 10,000 and 3 million. For the duplicated benchmarks, the same random values are repeated an average of 5 times.

The erasure benchmarks erase all n elements randomly until the container is empty.

The successful lookup benchmarks are done by looking up all n values, in their original insertion order.

The unsuccessful lookup benchmarks use n randomly generated integers but using a different seed value.

GCC 11 + libstdc++-v3, x64

Insertion
running insertion.xlsx.practice
running%20insertion.xlsx.practice non unique
running%20insertion.xlsx.practice non unique 5

non-duplicate elements

duplicate elements

duplicate elements
max load factor 5
running%20insertion.xlsx.practice norehash
running%20insertion.xlsx.practice norehash non unique
running%20insertion.xlsx.practice norehash non unique 5

non-duplicate elements,
prior reserve

duplicate elements,
prior reserve

duplicate elements,
max load factor 5,
prior reserve
Erasure
scattered%20erasure.xlsx.practice
scattered%20erasure.xlsx.practice non unique
scattered%20erasure.xlsx.practice non unique 5

non-duplicate elements

duplicate elements

duplicate elements
max load factor 5
Successful Lookup
scattered%20successful%20looukp.xlsx.practice
scattered%20successful%20looukp.xlsx.practice non unique
scattered%20successful%20looukp.xlsx.practice non unique 5

non-duplicate elements

duplicate elements

duplicate elements,
max load factor 5
Unsuccessful lookup
scattered%20unsuccessful%20looukp.xlsx.practice
scattered%20unsuccessful%20looukp.xlsx.practice non unique
scattered%20unsuccessful%20looukp.xlsx.practice non unique 5

non-duplicate elements

duplicate elements

duplicate elements,
max load factor 5

Clang 12 + libc++, x64

Insertion
running%20insertion.xlsx.practice
running%20insertion.xlsx.practice non unique
running%20insertion.xlsx.practice non unique 5

non-duplicate elements

duplicate elements

duplicate elements,
max load factor 5
running%20insertion.xlsx.practice norehash
running%20insertion.xlsx.practice norehash non unique
running%20insertion.xlsx.practice norehash non unique 5

non-duplicate elements,
prior reserve

duplicate elements,
prior reserve

duplicate elements,
max load factor 5,
prior reserve
Erasure
scattered%20erasure.xlsx.practice
scattered%20erasure.xlsx.practice non unique
scattered%20erasure.xlsx.practice non unique 5

non-duplicate elements

duplicate elements

duplicate elements,
max load factor 5
Successful lookup
scattered%20successful%20looukp.xlsx.practice
scattered%20successful%20looukp.xlsx.practice non unique
scattered%20successful%20looukp.xlsx.practice non unique 5

non-duplicate elements

duplicate elements

duplicate elements,
max load factor 5
Unsuccessful lookup
scattered%20unsuccessful%20looukp.xlsx.practice
scattered%20unsuccessful%20looukp.xlsx.practice non unique
scattered%20unsuccessful%20looukp.xlsx.practice non unique 5

non-duplicate elements

duplicate elements

duplicate elements,
max load factor 5

Visual Studio 2019 + Dinkumware, x64

Insertion
running%20insertion.xlsx.practice
running%20insertion.xlsx.practice non unique
running%20insertion.xlsx.practice non unique 5

non-duplicate elements

duplicate elements

duplicate elements,
max load factor 5
running%20insertion.xlsx.practice norehash
running%20insertion.xlsx.practice norehash non unique
running%20insertion.xlsx.practice norehash non unique 5

non-duplicate elements,
prior reserve

duplicate elements,
prior reserve

duplicate elements,
max load factor 5,
prior reserve
Erasure
scattered%20erasure.xlsx.practice
scattered%20erasure.xlsx.practice non unique
scattered%20erasure.xlsx.practice non unique 5

non-duplicate elements

duplicate elements

duplicate elements,
max load factor 5
Successful lookup
scattered%20successful%20looukp.xlsx.practice
scattered%20successful%20looukp.xlsx.practice non unique
scattered%20successful%20looukp.xlsx.practice non unique 5

non-duplicate elements

duplicate elements

duplicate elements,
max load factor 5
Unsuccessful lookup
scattered%20unsuccessful%20looukp.xlsx.practice
scattered%20unsuccessful%20looukp.xlsx.practice non unique
scattered%20unsuccessful%20looukp.xlsx.practice non unique 5

non-duplicate elements

duplicate elements

duplicate elements,
max load factor 5

boost::unordered_(flat|node)_map

All benchmarks were created using:

  • absl::flat_hash_map<uint64_t, uint64_t>

  • boost::unordered_map<uint64_t, uint64_t>

  • boost::unordered_flat_map<uint64_t, uint64_t>

  • boost::unordered_node_map<uint64_t, uint64_t>

The source code can be found here.

The insertion benchmarks insert n random values, where n is between 10,000 and 10 million.

The erasure benchmarks erase traverse the n elements and erase those with odd key (50% on average).

The successful lookup benchmarks are done by looking up all n values, in their original insertion order.

The unsuccessful lookup benchmarks use n randomly generated integers but using a different seed value.

GCC 11, x64

Running%20insertion.xlsx.plot
Running%20erasure.xlsx.plot
Scattered%20successful%20looukp.xlsx.plot
Scattered%20unsuccessful%20looukp.xlsx.plot

running insertion

running erasure

successful lookup

unsuccessful lookup

Clang 12, x64

Running%20insertion.xlsx.plot
Running%20erasure.xlsx.plot
Scattered%20successful%20looukp.xlsx.plot
Scattered%20unsuccessful%20looukp.xlsx.plot

running insertion

running erasure

successful lookup

unsuccessful lookup

Visual Studio 2019, x64

Running%20insertion.xlsx.plot
Running%20erasure.xlsx.plot
Scattered%20successful%20looukp.xlsx.plot
Scattered%20unsuccessful%20looukp.xlsx.plot

running insertion

running erasure

successful lookup

unsuccessful lookup

Clang 12, ARM64

Running%20insertion.xlsx.plot
Running%20erasure.xlsx.plot
Scattered%20successful%20looukp.xlsx.plot
Scattered%20unsuccessful%20looukp.xlsx.plot

running insertion

running erasure

successful lookup

unsuccessful lookup

GCC 11, x86

Running%20insertion.xlsx.plot
Running%20erasure.xlsx.plot
Scattered%20successful%20looukp.xlsx.plot
Scattered%20unsuccessful%20looukp.xlsx.plot

running insertion

running erasure

successful lookup

unsuccessful lookup

Clang 12, x86

Running%20insertion.xlsx.plot
Running%20erasure.xlsx.plot
Scattered%20successful%20looukp.xlsx.plot
Scattered%20unsuccessful%20looukp.xlsx.plot

running insertion

running erasure

successful lookup

unsuccessful lookup

Visual Studio 2019, x86

Running%20insertion.xlsx.plot
Running%20erasure.xlsx.plot
Scattered%20successful%20looukp.xlsx.plot
Scattered%20unsuccessful%20looukp.xlsx.plot

running insertion

running erasure

successful lookup

unsuccessful lookup

Implementation Rationale

Closed-addressing containers

boost::unordered_[multi]set and boost::unordered_[multi]map adhere to the standard requirements for unordered associative containers, so the interface was fixed. But there are still some implementation decisions to make. The priorities are conformance to the standard and portability.

The Wikipedia article on hash tables has a good summary of the implementation issues for hash tables in general.

Data Structure

By specifying an interface for accessing the buckets of the container the standard pretty much requires that the hash table uses closed addressing.

It would be conceivable to write a hash table that uses another method. For example, it could use open addressing, and use the lookup chain to act as a bucket but there are some serious problems with this:

  • The standard requires that pointers to elements aren’t invalidated, so the elements can’t be stored in one array, but will need a layer of indirection instead - losing the efficiency and most of the memory gain, the main advantages of open addressing.

  • Local iterators would be very inefficient and may not be able to meet the complexity requirements.

  • There are also the restrictions on when iterators can be invalidated. Since open addressing degrades badly when there are a high number of collisions the restrictions could prevent a rehash when it’s really needed. The maximum load factor could be set to a fairly low value to work around this - but the standard requires that it is initially set to 1.0.

  • And since the standard is written with a eye towards closed addressing, users will be surprised if the performance doesn’t reflect that.

So closed addressing is used.

Number of Buckets

There are two popular methods for choosing the number of buckets in a hash table. One is to have a prime number of buckets, another is to use a power of 2.

Using a prime number of buckets, and choosing a bucket by using the modulus of the hash function’s result will usually give a good result. The downside is that the required modulus operation is fairly expensive. This is what the containers used to do in most cases.

Using a power of 2 allows for much quicker selection of the bucket to use, but at the expense of losing the upper bits of the hash value. For some specially designed hash functions it is possible to do this and still get a good result but as the containers can take arbitrary hash functions this can’t be relied on.

To avoid this a transformation could be applied to the hash function, for an example see Thomas Wang’s article on integer hash functions. Unfortunately, a transformation like Wang’s requires knowledge of the number of bits in the hash value, so it was only used when size_t was 64 bit.

Since release 1.79.0, Fibonacci hashing is used instead. With this implementation, the bucket number is determined by using (h * m) >> (w - k), where h is the hash value, m is 2^w divided by the golden ratio, w is the word size (32 or 64), and 2^k is the number of buckets. This provides a good compromise between speed and distribution.

Since release 1.80.0, prime numbers are chosen for the number of buckets in tandem with sophisticated modulo arithmetic. This removes the need for "mixing" the result of the user’s hash function as was used for release 1.79.0.

Open-addresing containers

The C++ standard specification of unordered associative containers impose severe limitations on permissible implementations, the most important being that closed addressing is implicitly assumed. Slightly relaxing this specification opens up the possibility of providing container variations taking full advantage of open-addressing techniques.

The design of boost::unordered_flat_set/unordered_node_set and boost::unordered_flat_map/unordered_node_map has been guided by Peter Dimov’s Development Plan for Boost.Unordered. We discuss here the most relevant principles.

Hash function

Given its rich functionality and cross-platform interoperability, boost::hash remains the default hash function of open-addressing containers. As it happens, boost::hash for integral and other basic types does not possess the statistical properties required by open addressing; to cope with this, we implement a post-mixing stage:

     a ← h mulx C,
     h ← high(a) xor low(a),

where mulx is an extended multiplication (128 bits in 64-bit architectures, 64 bits in 32-bit environments), and high and low are the upper and lower halves of an extended word, respectively. In 64-bit architectures, C is the integer part of 264∕φ, whereas in 32 bits C = 0xE817FB2Du has been obtained from Steele and Vigna (2021).

When using a hash function directly suitable for open addressing, post-mixing can be opted out by via a dedicated hash_is_avalanchingtrait. boost::hash specializations for string types are marked as avalanching.

Platform interoperability

The observable behavior of boost::unordered_flat_set/unordered_node_set and boost::unordered_flat_map/unordered_node_map is deterministically identical across different compilers as long as their std::size_types are the same size and the user-provided hash function and equality predicate are also interoperable —this includes elements being ordered in exactly the same way for the same sequence of operations.

Although the implementation internally uses SIMD technologies, such as SSE2 and Neon, when available, this does not affect interoperatility. For instance, the behavior is the same for Visual Studio on an x64-mode Intel CPU with SSE2 and for GCC on an IBM s390x without any supported SIMD technology.

Reference

Class template unordered_map

boost::unordered_map — An unordered associative container that associates unique keys with another value.

Synopsis

// #include <boost/unordered/unordered_map.hpp>

namespace boost {
  template<class Key,
           class T,
           class Hash = boost::hash<Key>,
           class Pred = std::equal_to<Key>,
           class Allocator = std::allocator<std::pair<const Key, T>>>
  class unordered_map {
  public:
    // types
    using key_type             = Key;
    using mapped_type          = T;
    using value_type           = std::pair<const Key, T>;
    using hasher               = Hash;
    using key_equal            = Pred;
    using allocator_type       = Allocator;
    using pointer              = typename boost::allocator_traits<Allocator>::pointer;
    using const_pointer        = typename boost::allocator_traits<Allocator>::const_pointer;
    using reference            = value_type&;
    using const_reference      = const value_type&;
    using size_type            = implementation-defined;
    using difference_type      = implementation-defined;

    using iterator             = implementation-defined;
    using const_iterator       = implementation-defined;
    using local_iterator       = implementation-defined;
    using const_local_iterator = implementation-defined;
    using node_type            = implementation-defined;
    using insert_return_type   = implementation-defined;

    // construct/copy/destroy
    unordered_map();
    explicit unordered_map(size_type n,
                           const hasher& hf = hasher(),
                           const key_equal& eql = key_equal(),
                           const allocator_type& a = allocator_type());
    template<class InputIterator>
      unordered_map(InputIterator f, InputIterator l,
                    size_type n = implementation-defined,
                    const hasher& hf = hasher(),
                    const key_equal& eql = key_equal(),
                    const allocator_type& a = allocator_type());
    unordered_map(const unordered_map& other);
    unordered_map(unordered_map&& other);
    template<class InputIterator>
      unordered_map(InputIterator f, InputIterator l, const allocator_type& a);
    explicit unordered_map(const Allocator& a);
    unordered_map(const unordered_map& other, const Allocator& a);
    unordered_map(unordered_map&& other, const Allocator& a);
    unordered_map(std::initializer_list<value_type> il,
                  size_type n = implementation-defined
                  const hasher& hf = hasher(),
                  const key_equal& eql = key_equal(),
                  const allocator_type& a = allocator_type());
    unordered_map(size_type n, const allocator_type& a);
    unordered_map(size_type n, const hasher& hf, const allocator_type& a);
    template<class InputIterator>
      unordered_map(InputIterator f, InputIterator l, size_type n, const allocator_type& a);
    template<class InputIterator>
      unordered_map(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                    const allocator_type& a);
    unordered_map(std::initializer_list<value_type> il, const allocator_type& a);
    unordered_map(std::initializer_list<value_type> il, size_type n, const allocator_type& a);
    unordered_map(std::initializer_list<value_type> il, size_type n, const hasher& hf,
                  const allocator_type& a);
    ~unordered_map();
    unordered_map& operator=(const unordered_map& other);
    unordered_map& operator=(unordered_map&& other)
      noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
               boost::is_nothrow_move_assignable_v<Hash> &&
               boost::is_nothrow_move_assignable_v<Pred>);
    unordered_map& operator=(std::initializer_list<value_type>);
    allocator_type get_allocator() const noexcept;

    // iterators
    iterator       begin() noexcept;
    const_iterator begin() const noexcept;
    iterator       end() noexcept;
    const_iterator end() const noexcept;
    const_iterator cbegin() const noexcept;
    const_iterator cend() const noexcept;

    // capacity
    [[nodiscard]] bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;

    // modifiers
    template<class... Args> std::pair<iterator, bool> emplace(Args&&... args);
    template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
    std::pair<iterator, bool> insert(const value_type& obj);
    std::pair<iterator, bool> insert(value_type&& obj);
    template<class P> std::pair<iterator, bool> insert(P&& obj);
    iterator       insert(const_iterator hint, const value_type& obj);
    iterator       insert(const_iterator hint, value_type&& obj);
    template<class P> iterator insert(const_iterator hint, P&& obj);
    template<class InputIterator> void insert(InputIterator first, InputIterator last);
    void insert(std::initializer_list<value_type>);

    template<class... Args>
      std::pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
    template<class... Args>
      std::pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
    template<class K, class... Args>
      std::pair<iterator, bool> try_emplace(K&& k, Args&&... args);
    template<class... Args>
      iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
    template<class... Args>
      iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);
    template<class K, class... Args>
      iterator try_emplace(const_iterator hint, K&& k, Args&&... args);
    template<class M>
      std::pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
    template<class M>
      std::pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
    template<class K, class M>
      std::pair<iterator, bool> insert_or_assign(K&& k, M&& obj);
    template<class M>
      iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
    template<class M>
      iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);
    template<class K, class M>
      iterator insert_or_assign(const_iterator hint, K&& k, M&& obj);

    node_type extract(const_iterator position);
    node_type extract(const key_type& k);
    template<class K> node_type extract(K&& k);
    insert_return_type insert(node_type&& nh);
    iterator           insert(const_iterator hint, node_type&& nh);

    iterator  erase(iterator position);
    iterator  erase(const_iterator position);
    size_type erase(const key_type& k);
    template<class K> size_type erase(K&& k);
    iterator  erase(const_iterator first, const_iterator last);
    void      quick_erase(const_iterator position);
    void      erase_return_void(const_iterator position);
    void      swap(unordered_map& other)
      noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
               boost::is_nothrow_swappable_v<Hash> &&
               boost::is_nothrow_swappable_v<Pred>);
    void      clear() noexcept;

    template<class H2, class P2>
      void merge(unordered_map<Key, T, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_map<Key, T, H2, P2, Allocator>&& source);
    template<class H2, class P2>
      void merge(unordered_multimap<Key, T, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_multimap<Key, T, H2, P2, Allocator>&& source);

    // observers
    hasher hash_function() const;
    key_equal key_eq() const;

    // map operations
    iterator         find(const key_type& k);
    const_iterator   find(const key_type& k) const;
    template<class K>
      iterator       find(const K& k);
    template<class K>
      const_iterator find(const K& k) const;
    template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
      iterator       find(CompatibleKey const& k, CompatibleHash const& hash,
                          CompatiblePredicate const& eq);
    template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
      const_iterator find(CompatibleKey const& k, CompatibleHash const& hash,
                          CompatiblePredicate const& eq) const;
    size_type        count(const key_type& k) const;
    template<class K>
      size_type      count(const K& k) const;
    bool             contains(const key_type& k) const;
    template<class K>
      bool           contains(const K& k) const;
    std::pair<iterator, iterator>               equal_range(const key_type& k);
    std::pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
    template<class K>
      std::pair<iterator, iterator>             equal_range(const K& k);
    template<class K>
      std::pair<const_iterator, const_iterator> equal_range(const K& k) const;

    // element access
    mapped_type& operator[](const key_type& k);
    mapped_type& operator[](key_type&& k);
    template<class K> mapped_type& operator[](K&& k);
    mapped_type& at(const key_type& k);
    const mapped_type& at(const key_type& k) const;
    template<class K> mapped_type& at(const K& k);
    template<class K> const mapped_type& at(const K& k) const;

    // bucket interface
    size_type bucket_count() const noexcept;
    size_type max_bucket_count() const noexcept;
    size_type bucket_size(size_type n) const;
    size_type bucket(const key_type& k) const;
    template<class K> size_type bucket(const K& k) const;
    local_iterator begin(size_type n);
    const_local_iterator begin(size_type n) const;
    local_iterator end(size_type n);
    const_local_iterator end(size_type n) const;
    const_local_iterator cbegin(size_type n) const;
    const_local_iterator cend(size_type n) const;

    // hash policy
    float load_factor() const noexcept;
    float max_load_factor() const noexcept;
    void max_load_factor(float z);
    void rehash(size_type n);
    void reserve(size_type n);
  };

  // Deduction Guides
  template<class InputIterator,
           class Hash = boost::hash<iter-key-type<InputIterator>>,
           class Pred = std::equal_to<iter-key-type<InputIterator>>,
           class Allocator = std::allocator<iter-to-alloc-type<InputIterator>>>
    unordered_map(InputIterator, InputIterator, typename see below::size_type = see below,
                  Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Hash, Pred,
                       Allocator>;

  template<class Key, class T, class Hash = boost::hash<Key>,
           class Pred = std::equal_to<Key>,
           class Allocator = std::allocator<std::pair<const Key, T>>>
    unordered_map(std::initializer_list<std::pair<Key, T>>,
                  typename see below::size_type = see below, Hash = Hash(),
                  Pred = Pred(), Allocator = Allocator())
      -> unordered_map<Key, T, Hash, Pred, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_map(InputIterator, InputIterator, typename see below::size_type, Allocator)
      -> unordered_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
                       boost::hash<iter-key-type<InputIterator>>,
                       std::equal_to<iter-key-type<InputIterator>>, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_map(InputIterator, InputIterator, Allocator)
      -> unordered_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
                       boost::hash<iter-key-type<InputIterator>>,
                       std::equal_to<iter-key-type<InputIterator>>, Allocator>;

  template<class InputIterator, class Hash, class Allocator>
    unordered_map(InputIterator, InputIterator, typename see below::size_type, Hash, Allocator)
      -> unordered_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Hash,
                       std::equal_to<iter-key-type<InputIterator>>, Allocator>;

  template<class Key, class T, class Allocator>
    unordered_map(std::initializer_list<std::pair<Key, T>>, typename see below::size_type,
                  Allocator)
      -> unordered_map<Key, T, boost::hash<Key>, std::equal_to<Key>, Allocator>;

  template<class Key, class T, class Allocator>
    unordered_map(std::initializer_list<std::pair<Key, T>>, Allocator)
      -> unordered_map<Key, T, boost::hash<Key>, std::equal_to<Key>, Allocator>;

  template<class Key, class T, class Hash, class Allocator>
    unordered_map(std::initializer_list<std::pair<Key, T>>, typename see below::size_type, Hash,
                  Allocator)
      -> unordered_map<Key, T, Hash, std::equal_to<Key>, Allocator>;

  // Equality Comparisons
  template<class Key, class T, class Hash, class Pred, class Alloc>
    bool operator==(const unordered_map<Key, T, Hash, Pred, Alloc>& x,
                    const unordered_map<Key, T, Hash, Pred, Alloc>& y);

  template<class Key, class T, class Hash, class Pred, class Alloc>
    bool operator!=(const unordered_map<Key, T, Hash, Pred, Alloc>& x,
                    const unordered_map<Key, T, Hash, Pred, Alloc>& y);

  // swap
  template<class Key, class T, class Hash, class Pred, class Alloc>
    void swap(unordered_map<Key, T, Hash, Pred, Alloc>& x,
              unordered_map<Key, T, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));

  template<class K, class T, class H, class P, class A, class Predicate>
    typename unordered_map<K, T, H, P, A>::size_type
       erase_if(unordered_map<K, T, H, P, A>& c, Predicate pred);
}

Description

Template Parameters

Key

Key must be Erasable from the container (i.e. allocator_traits can destroy it).

T

T must be Erasable from the container (i.e. allocator_traits can destroy it).

Hash

A unary function object type that acts a hash function for a Key. It takes a single argument of type Key and returns a value of type std::size_t.

Pred

A binary function object that implements an equivalence relation on values of type Key. A binary function object that induces an equivalence relation on values of type Key. It takes two arguments of type Key and returns a value of type bool.

Allocator

An allocator whose value type is the same as the container’s value type.

The elements are organized into buckets. Keys with the same hash code are stored in the same bucket.

The number of buckets can be automatically increased by a call to insert, or as the result of calling rehash.

Typedefs

typedef typename allocator_type::pointer pointer;

value_type* if allocator_type::pointer is not defined.

typedef typename allocator_type::const_pointer const_pointer;

boost::pointer_to_other<pointer, value_type>::type if allocator_type::const_pointer is not defined.

typedef implementation-defined size_type;

An unsigned integral type.

size_type can represent any non-negative value of difference_type.

typedef implementation-defined difference_type;

A signed integral type.

Is identical to the difference type of iterator and const_iterator.

typedef implementation-defined iterator;

An iterator whose value type is value_type.

The iterator category is at least a forward iterator.

Convertible to const_iterator.

typedef implementation-defined const_iterator;

A constant iterator whose value type is value_type.

The iterator category is at least a forward iterator.

typedef implementation-defined local_iterator;

An iterator with the same value type, difference type and pointer and reference type as iterator.

A local_iterator object can be used to iterate through a single bucket.

typedef implementation-defined const_local_iterator;

A constant iterator with the same value type, difference type and pointer and reference type as const_iterator.

A const_local_iterator object can be used to iterate through a single bucket.

typedef implementation-defined node_type;

See node_handle_map for details.

typedef implementation-defined insert_return_type;

Structure returned by inserting node_type.

Constructors

Default Constructor
unordered_map();

Constructs an empty container using hasher() as the hash function, key_equal() as the key equality predicate, allocator_type() as the allocator and a maximum load factor of 1.0.

Postconditions:

size() == 0
Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Bucket Count Constructor
explicit unordered_map(size_type n,
                       const hasher& hf = hasher(),
                       const key_equal& eql = key_equal(),
                       const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate, a as the allocator and a maximum load factor of 1.0.

Postconditions:

size() == 0
Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Iterator Range Constructor
template<class InputIterator>
  unordered_map(InputIterator f, InputIterator l,
                size_type n = implementation-defined,
                const hasher& hf = hasher(),
                const key_equal& eql = key_equal(),
                const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate, a as the allocator and a maximum load factor of 1.0 and inserts the elements from [f, l) into it.

Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Copy Constructor
unordered_map(unordered_map const& other);

The copy constructor. Copies the contained elements, hash function, predicate, maximum load factor and allocator.

If Allocator::select_on_container_copy_construction exists and has the right signature, the allocator will be constructed from its result.

Requires:

value_type is copy constructible
Move Constructor
unordered_map(unordered_map&& other);

The move constructor.

Notes:

This is implemented using Boost.Move.
Requires:

value_type is move-constructible.

On compilers without rvalue reference support the emulation does not support moving without calling boost::move if value_type is not copyable. So, for example, you can’t return the container from a function.
Iterator Range Constructor with Allocator
template<class InputIterator>
  unordered_map(InputIterator f, InputIterator l, const allocator_type& a);

Constructs an empty container using a as the allocator, with the default hash function and key equality predicate and a maximum load factor of 1.0 and inserts the elements from [f, l) into it.

Requires:

hasher, key_equal need to be DefaultConstructible.
Allocator Constructor
explicit unordered_map(Allocator const& a);

Constructs an empty container, using allocator a.

Copy Constructor with Allocator
unordered_map(unordered_map const& other, Allocator const& a);

Constructs an container, copying other's contained elements, hash function, predicate, maximum load factor, but using allocator a.

Move Constructor with Allocator
unordered_map(unordered_map&& other, Allocator const& a);

Construct a container moving other's contained elements, and having the hash function, predicate and maximum load factor, but using allocate a.

Notes:

This is implemented using Boost.Move.
Requires:

value_type is move insertable.
Initializer List Constructor
unordered_map(std::initializer_list<value_type> il,
              size_type n = implementation-defined
              const hasher& hf = hasher(),
              const key_equal& eql = key_equal(),
              const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate, a as the allocator and a maximum load factor of 1.0 and inserts the elements from il into it.

Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Bucket Count Constructor with Allocator
unordered_map(size_type n, allocator_type const& a);

Constructs an empty container with at least n buckets, using hf as the hash function, the default hash function and key equality predicate, a as the allocator and a maximum load factor of 1.0.

Postconditions:

size() == 0
Requires:

hasher and key_equal need to be DefaultConstructible.
Bucket Count Constructor with Hasher and Allocator
unordered_map(size_type n, hasher const& hf, allocator_type const& a);

Constructs an empty container with at least n buckets, using hf as the hash function, the default key equality predicate, a as the allocator and a maximum load factor of 1.0.

Postconditions:

size() == 0
Requires:

key_equal needs to be DefaultConstructible.
Iterator Range Constructor with Bucket Count and Allocator
template<class InputIterator>
  unordered_map(InputIterator f, InputIterator l, size_type n, const allocator_type& a);

Constructs an empty container with at least n buckets, using a as the allocator, with the default hash function and key equality predicate and a maximum load factor of 1.0 and inserts the elements from [f, l) into it.

Requires:

hasher, key_equal need to be DefaultConstructible.
Iterator Range Constructor with Bucket Count and Hasher
    template<class InputIterator>
      unordered_map(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                    const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, a as the allocator, with the default key equality predicate and a maximum load factor of 1.0 and inserts the elements from [f, l) into it.

Requires:

key_equal needs to be DefaultConstructible.
initializer_list Constructor with Allocator
unordered_map(std::initializer_list<value_type> il, const allocator_type& a);

Constructs an empty container using a as the allocator and a maximum load factor of 1.0 and inserts the elements from il into it.

Requires:

hasher and key_equal need to be DefaultConstructible.
initializer_list Constructor with Bucket Count and Allocator
unordered_map(std::initializer_list<value_type> il, size_type n, const allocator_type& a);

Constructs an empty container with at least n buckets, using a as the allocator and a maximum load factor of 1.0 and inserts the elements from il into it.

Requires:

hasher and key_equal need to be DefaultConstructible.
initializer_list Constructor with Bucket Count and Hasher and Allocator
unordered_map(std::initializer_list<value_type> il, size_type n, const hasher& hf,
              const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, a as the allocator and a maximum load factor of 1.0 and inserts the elements from il into it.

Requires:

key_equal needs to be DefaultConstructible.

Destructor

~unordered_map();
Note:

The destructor is applied to every element, and all memory is deallocated

Assignment

Copy Assignment
unordered_map& operator=(unordered_map const& other);

The assignment operator. Copies the contained elements, hash function, predicate and maximum load factor but not the allocator.

If Alloc::propagate_on_container_copy_assignment exists and Alloc::propagate_on_container_copy_assignment::value is true, the allocator is overwritten, if not the copied elements are created using the existing allocator.

Requires:

value_type is copy constructible
Move Assignment
unordered_map& operator=(unordered_map&& other)
  noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
           boost::is_nothrow_move_assignable_v<Hash> &&
           boost::is_nothrow_move_assignable_v<Pred>);

The move assignment operator.

If Alloc::propagate_on_container_move_assignment exists and Alloc::propagate_on_container_move_assignment::value is true, the allocator is overwritten, if not the moved elements are created using the existing allocator.

Notes:

On compilers without rvalue references, this is emulated using Boost.Move. Note that on some compilers the copy assignment operator may be used in some circumstances.
Requires:

value_type is move constructible.
Initializer List Assignment
unordered_map& operator=(std::initializer_list<value_type> il);

Assign from values in initializer list. All existing elements are either overwritten by the new elements or destroyed.

Requires:

value_type is CopyInsertable into the container and CopyAssignable.

Iterators

begin
iterator begin() noexcept;
const_iterator begin() const noexcept;
Returns:

An iterator referring to the first element of the container, or if the container is empty the past-the-end value for the container.
end
iterator end() noexcept;
const_iterator end() const noexcept;
Returns:

An iterator which refers to the past-the-end value for the container.
cbegin
const_iterator cbegin() const noexcept;
Returns:

A const_iterator referring to the first element of the container, or if the container is empty the past-the-end value for the container.
cend
const_iterator cend() const noexcept;
Returns:

A const_iterator which refers to the past-the-end value for the container.

Size and Capacity

empty
[[nodiscard]] bool empty() const noexcept;
Returns:

size() == 0
size
size_type size() const noexcept;
Returns:

std::distance(begin(), end())
max_size
size_type max_size() const noexcept;
Returns:

size() of the largest possible container.

Modifiers

emplace
template<class... Args> std::pair<iterator, bool> emplace(Args&&... args);

Inserts an object, constructed with the arguments args, in the container if and only if there is no element in the container with an equivalent key.

Requires:

value_type is EmplaceConstructible into X from args.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

If the compiler doesn’t support variadic template arguments or rvalue references, this is emulated for up to 10 arguments, with no support for rvalue references or move semantics.

Since existing std::pair implementations don’t support std::piecewise_construct this emulates it, but using boost::unordered::piecewise_construct.
emplace_hint
    template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);

Inserts an object, constructed with the arguments args, in the container if and only if there is no element in the container with an equivalent key.

position is a suggestion to where the element should be inserted.

Requires:

value_type is EmplaceConstructible into X from args.
Returns:

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

If the compiler doesn’t support variadic template arguments or rvalue references, this is emulated for up to 10 arguments, with no support for rvalue references or move semantics.

Since existing std::pair implementations don’t support std::piecewise_construct this emulates it, but using boost::unordered::piecewise_construct.
Copy Insert
std::pair<iterator, bool> insert(const value_type& obj);

Inserts obj in the container if and only if there is no element in the container with an equivalent key.

Requires:

value_type is CopyInsertable.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Move Insert
std::pair<iterator, bool> insert(value_type&& obj);

Inserts obj in the container if and only if there is no element in the container with an equivalent key.

Requires:

value_type is MoveInsertable.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Emplace Insert
template<class P> std::pair<iterator, bool> insert(P&& obj);

Inserts an element into the container by performing emplace(std::forward<P>(value)).

Only participates in overload resolution if std::is_constructible<value_type, P&&>::value is true.

Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Copy Insert with Hint
iterator insert(const_iterator hint, const value_type& obj);

Inserts obj in the container if and only if there is no element in the container with an equivalent key.

hint is a suggestion to where the element should be inserted.

Requires:

value_type is CopyInsertable.
Returns:

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Move Insert with Hint
iterator insert(const_iterator hint, value_type&& obj);

Inserts obj in the container if and only if there is no element in the container with an equivalent key.

hint is a suggestion to where the element should be inserted.

Requires:

value_type is MoveInsertable.
Returns:

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Emplace Insert with Hint
template<class P> iterator insert(const_iterator hint, P&& obj);

Inserts an element into the container by performing emplace_hint(hint, std::forward<P>(value)).

Only participates in overload resolution if std::is_constructible<value_type, P&&>::value is true.

hint is a suggestion to where the element should be inserted.

Returns:

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Insert Iterator Range
template<class InputIterator> void insert(InputIterator first, InputIterator last);

Inserts a range of elements into the container. Elements are inserted if and only if there is no element in the container with an equivalent key.

Requires:

value_type is EmplaceConstructible into X from *first.
Throws:

When inserting a single element, if an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Insert Initializer List
void insert(std::initializer_list<value_type>);

Inserts a range of elements into the container. Elements are inserted if and only if there is no element in the container with an equivalent key.

Requires:

value_type is EmplaceConstructible into X from *first.
Throws:

When inserting a single element, if an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
try_emplace
template<class... Args>
  std::pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
template<class... Args>
  std::pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
template <class K, class... Args>
  std::pair<iterator, bool> try_emplace(K&& k, Args&&... args)

Inserts a new node into the container if there is no existing element with key k contained within it.

If there is an existing element with key k this function does nothing.

Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

This function is similiar to emplace except the value_type is constructed using:

// first two overloads
value_type(std::piecewise_construct,
           std::forward_as_tuple(boost::forward<Key>(k)),
           std::forward_as_tuple(boost::forward<Args>(args)...))

// third overload
value_type(std::piecewise_construct,
           std::forward_as_tuple(boost::forward<K>(k)),
           std::forward_as_tuple(boost::forward<Args>(args)...))

instead of emplace which simply forwards all arguments to value_type's constructor.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

The template <class K, class…​ Args> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.

If the compiler doesn’t support variadic template arguments or rvalue references, this is emulated for up to 10 arguments, with no support for rvalue references or move semantics.

Since existing std::pair implementations don’t support std::piecewise_construct this emulates it, but using boost::unordered::piecewise_construct.
try_emplace with Hint
template<class... Args>
  iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
template<class... Args>
  iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);
template<class K, class... Args>
  iterator try_emplace(const_iterator hint, K&& k, Args&&... args);

Inserts a new node into the container if there is no existing element with key k contained within it.

If there is an existing element with key k this function does nothing.

hint is a suggestion to where the element should be inserted.

Returns:

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

This function is similiar to emplace_hint except the value_type is constructed using:

// first two overloads
value_type(std::piecewise_construct,
           std::forward_as_tuple(boost::forward<Key>(k)),
           std::forward_as_tuple(boost::forward<Args>(args)...))

// third overload
value_type(std::piecewise_construct,
           std::forward_as_tuple(boost::forward<K>(k)),
           std::forward_as_tuple(boost::forward<Args>(args)...))

instead of emplace_hint which simply forwards all arguments to value_type's constructor.

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

The template <class K, class…​ Args> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.

If the compiler doesn’t support variadic template arguments or rvalue references, this is emulated for up to 10 arguments, with no support for rvalue references or move semantics.

Since existing std::pair implementations don’t support std::piecewise_construct this emulates it, but using boost::unordered::piecewise_construct.
insert_or_assign
template<class M>
  std::pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
template<class M>
  std::pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
template<class K, class M>
  std::pair<iterator, bool> insert_or_assign(K&& k, M&& obj);

Inserts a new element into the container or updates an existing one by assigning to the contained value.

If there is an element with key k, then it is updated by assigning boost::forward<M>(obj).

If there is no such element, it is added to the container as:

// first two overloads
value_type(std::piecewise_construct,
           std::forward_as_tuple(boost::forward<Key>(k)),
           std::forward_as_tuple(boost::forward<M>(obj)))

// third overload
value_type(std::piecewise_construct,
           std::forward_as_tuple(boost::forward<K>(k)),
           std::forward_as_tuple(boost::forward<M>(obj)))
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

The template<class K, class M> only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
insert_or_assign with Hint
template<class M>
  iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
template<class M>
  iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);
template<class K, class M>
  iterator insert_or_assign(const_iterator hint, K&& k, M&& obj);

Inserts a new element into the container or updates an existing one by assigning to the contained value.

If there is an element with key k, then it is updated by assigning boost::forward<M>(obj).

If there is no such element, it is added to the container as:

// first two overloads
value_type(std::piecewise_construct,
           std::forward_as_tuple(boost::forward<Key>(k)),
           std::forward_as_tuple(boost::forward<M>(obj)))

// third overload
value_type(std::piecewise_construct,
           std::forward_as_tuple(boost::forward<K>(k)),
           std::forward_as_tuple(boost::forward<M>(obj)))

hint is a suggestion to where the element should be inserted.

Returns:

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

The template<class K, class M> only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
Extract by Iterator
node_type extract(const_iterator position);

Removes the element pointed to by position.

Returns:

A node_type owning the element.
Notes:

A node extracted using this method can be inserted into a compatible unordered_multimap.
Extract by Key
node_type extract(const key_type& k);
template<class K> node_type extract(K&& k);

Removes an element with key equivalent to k.

Returns:

A node_type owning the element if found, otherwise an empty node_type.
Throws:

Only throws an exception if it is thrown by hasher or key_equal.
Notes:

A node extracted using this method can be inserted into a compatible unordered_multimap.

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
Insert with node_handle
insert_return_type insert(node_type&& nh);

If nh is empty, has no effect.

Otherwise inserts the element owned by nh if and only if there is no element in the container with an equivalent key.

Requires:

nh is empty or nh.get_allocator() is equal to the container’s allocator.
Returns:

If nh was empty, returns an insert_return_type with: inserted equal to false, position equal to end() and node empty.

Otherwise if there was already an element with an equivalent key, returns an insert_return_type with: inserted equal to false, position pointing to a matching element and node contains the node from nh.

Otherwise if the insertion succeeded, returns an insert_return_type with: inserted equal to true, position pointing to the newly inserted element and node empty.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

This can be used to insert a node extracted from a compatible unordered_multimap.
Insert with Hint and node_handle
iterator insert(const_iterator hint, node_type&& nh);

If nh is empty, has no effect.

Otherwise inserts the element owned by nh if and only if there is no element in the container with an equivalent key.

If there is already an element in the container with an equivalent key has no effect on nh (i.e. nh still contains the node.)

hint is a suggestion to where the element should be inserted.

Requires:

nh is empty or nh.get_allocator() is equal to the container’s allocator.
Returns:

If nh was empty returns end().

If there was already an element in the container with an equivalent key returns an iterator pointing to that.

Otherwise returns an iterator pointing to the newly inserted element.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

This can be used to insert a node extracted from a compatible unordered_multimap.
Erase by Position
iterator erase(iterator position);
iterator erase(const_iterator position);

Erase the element pointed to by position.

Returns:

The iterator following position before the erasure.
Throws:

Only throws an exception if it is thrown by hasher or key_equal.
Notes:

In older versions this could be inefficient because it had to search through several buckets to find the position of the returned iterator. The data structure has been changed so that this is no longer the case, and the alternative erase methods have been deprecated.
Erase by Key
size_type erase(const key_type& k);
template<class K> size_type erase(K&& k);

Erase all elements with key equivalent to k.

Returns:

The number of elements erased.
Throws:

Only throws an exception if it is thrown by hasher or key_equal.
Notes:

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
Erase Range
iterator erase(const_iterator first, const_iterator last);

Erases the elements in the range from first to last.

Returns:

The iterator following the erased elements - i.e. last.
Throws:

Only throws an exception if it is thrown by hasher or key_equal.

In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations.
quick_erase
void quick_erase(const_iterator position);

Erase the element pointed to by position.

Throws:

Only throws an exception if it is thrown by hasher or key_equal.

In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations.
Notes:

This method was implemented because returning an iterator to the next element from erase was expensive, but the container has been redesigned so that is no longer the case. So this method is now deprecated.
erase_return_void
void erase_return_void(const_iterator position);

Erase the element pointed to by position.

Throws:

Only throws an exception if it is thrown by hasher or key_equal.

In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations.
Notes:

This method was implemented because returning an iterator to the next element from erase was expensive, but the container has been redesigned so that is no longer the case. So this method is now deprecated.
swap
void swap(unordered_map& other)
  noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
           boost::is_nothrow_swappable_v<Hash> &&
           boost::is_nothrow_swappable_v<Pred>);

Swaps the contents of the container with the parameter.

If Allocator::propagate_on_container_swap is declared and Allocator::propagate_on_container_swap::value is true then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.

Throws:

Doesn’t throw an exception unless it is thrown by the copy constructor or copy assignment operator of key_equal or hasher.
Notes:

The exception specifications aren’t quite the same as the C++11 standard, as the equality predicate and hash function are swapped using their copy constructors.
clear
void clear();

Erases all elements in the container.

Postconditions:

size() == 0
Throws:

Never throws an exception.
merge
template<class H2, class P2>
  void merge(unordered_map<Key, T, H2, P2, Allocator>& source);
template<class H2, class P2>
  void merge(unordered_map<Key, T, H2, P2, Allocator>&& source);
template<class H2, class P2>
  void merge(unordered_multimap<Key, T, H2, P2, Allocator>& source);
template<class H2, class P2>
  void merge(unordered_multimap<Key, T, H2, P2, Allocator>&& source);

Attempt to "merge" two containers by iterating source and extracting any node in source that is not contained in *this and then inserting it into *this.

Because source can have a different hash function and key equality predicate, the key of each node in source is rehashed using this->hash_function() and then, if required, compared using this->key_eq().

The behavior of this function is undefined if this->get_allocator() != source.get_allocator().

This function does not copy or move any elements and instead simply relocates the nodes from source into *this.

Notes:

  • Pointers and references to transferred elements remain valid.

  • Invalidates iterators to transferred elements.

  • Invalidates iterators belonging to *this.

  • Iterators to non-transferred elements in source remain valid.

Observers

get_allocator
allocator_type get_allocator() const;
hash_function
hasher hash_function() const;
Returns:

The container’s hash function.
key_eq
key_equal key_eq() const;
Returns:

The container’s key equality predicate

Lookup

find
iterator         find(const key_type& k);
const_iterator   find(const key_type& k) const;
template<class K>
  iterator       find(const K& k);
template<class K>
  const_iterator find(const K& k) const;
template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
  iterator       find(CompatibleKey const& k, CompatibleHash const& hash,
                      CompatiblePredicate const& eq);
template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
  const_iterator find(CompatibleKey const& k, CompatibleHash const& hash,
                      CompatiblePredicate const& eq) const;
Returns:

An iterator pointing to an element with key equivalent to k, or b.end() if no such element exists.
Notes:

The templated overloads containing CompatibleKey, CompatibleHash and CompatiblePredicate are non-standard extensions which allow you to use a compatible hash function and equality predicate for a key of a different type in order to avoid an expensive type cast. In general, its use is not encouraged and instead the K member function templates should be used.

The template <typename K> overloads only participate in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
count
size_type        count(const key_type& k) const;
template<class K>
  size_type      count(const K& k) const;
Returns:

The number of elements with key equivalent to k.
Notes:

The template <typename K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
contains
bool             contains(const key_type& k) const;
template<class K>
  bool           contains(const K& k) const;
Returns:

A boolean indicating whether or not there is an element with key equal to key in the container
Notes:

The template <typename K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
equal_range
std::pair<iterator, iterator>               equal_range(const key_type& k);
std::pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
template<class K>
  std::pair<iterator, iterator>             equal_range(const K& k);
template<class K>
  std::pair<const_iterator, const_iterator> equal_range(const K& k) const;
Returns:

A range containing all elements with key equivalent to k. If the container doesn’t contain any such elements, returns std::make_pair(b.end(), b.end()).
Notes:

The template <typename K> overloads only participate in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
operator[]
mapped_type& operator[](const key_type& k);
mapped_type& operator[](key_type&& k);
template<class K> mapped_type& operator[](K&& k);
Effects:

If the container does not already contain an elements with a key equivalent to k, inserts the value std::pair<key_type const, mapped_type>(k, mapped_type()).
Returns:

A reference to x.second where x is the element already in the container, or the newly inserted element with a key equivalent to k.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
at
mapped_type& at(const key_type& k);
const mapped_type& at(const key_type& k) const;
template<class K> mapped_type& at(const K& k);
template<class K> const mapped_type& at(const K& k) const;
Returns:

A reference to x.second where x is the (unique) element whose key is equivalent to k.
Throws:

An exception object of type std::out_of_range if no such element is present.
Notes:

The template<class K> overloads only participate in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.

Bucket Interface

bucket_count
size_type bucket_count() const noexcept;
Returns:

The number of buckets.
max_bucket_count
size_type max_bucket_count() const noexcept;
Returns:

An upper bound on the number of buckets.
bucket_size
size_type bucket_size(size_type n) const;
Requires:

n < bucket_count()
Returns:

The number of elements in bucket n.
bucket
size_type bucket(const key_type& k) const;
template<class K> size_type bucket(const K& k) const;
Returns:

The index of the bucket which would contain an element with key k.
Postconditions:

The return value is less than bucket_count().
Notes:

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
begin
local_iterator begin(size_type n);
const_local_iterator begin(size_type n) const;
Requires:

n shall be in the range [0, bucket_count()).
Returns:

A local iterator pointing the first element in the bucket with index n.
end
local_iterator end(size_type n);
const_local_iterator end(size_type n) const;
Requires:

n shall be in the range [0, bucket_count()).
Returns:

A local iterator pointing the 'one past the end' element in the bucket with index n.
cbegin
const_local_iterator cbegin(size_type n) const;
Requires:

n shall be in the range [0, bucket_count()).
Returns:

A constant local iterator pointing the first element in the bucket with index n.
cend
const_local_iterator cend(size_type n) const;
Requires:

n shall be in the range [0, bucket_count()).
Returns:

A constant local iterator pointing the 'one past the end' element in the bucket with index n.

Hash Policy

load_factor
float load_factor() const noexcept;
Returns:

The average number of elements per bucket.
max_load_factor
float max_load_factor() const noexcept;
Returns:

Returns the current maximum load factor.
Set max_load_factor
void max_load_factor(float z);
Effects:

Changes the container’s maximum load factor, using z as a hint.
rehash
void rehash(size_type n);

Changes the number of buckets so that there are at least n buckets, and so that the load factor is less than or equal to the maximum load factor. When applicable, this will either grow or shrink the bucket_count() associated with the container.

When size() == 0, rehash(0) will deallocate the underlying buckets array.

Invalidates iterators, and changes the order of elements. Pointers and references to elements are not invalidated.

Throws:

The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function.
reserve
void reserve(size_type n);

Equivalent to a.rehash(ceil(n / a.max_load_factor())), or a.rehash(1) if n > 0 and a.max_load_factor() == std::numeric_limits<float>::infinity().

Similar to rehash, this function can be used to grow or shrink the number of buckets in the container.

Invalidates iterators, and changes the order of elements. Pointers and references to elements are not invalidated.

Throws:

The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function.

Deduction Guides

A deduction guide will not participate in overload resolution if any of the following are true:

  • It has an InputIterator template parameter and a type that does not qualify as an input iterator is deduced for that parameter.

  • It has an Allocator template parameter and a type that does not qualify as an allocator is deduced for that parameter.

  • It has a Hash template parameter and an integral type or a type that qualifies as an allocator is deduced for that parameter.

  • It has a Pred template parameter and a type that qualifies as an allocator is deduced for that parameter.

A size_­type parameter type in a deduction guide refers to the size_­type member type of the container type deduced by the deduction guide. Its default value coincides with the default value of the constructor selected.

iter-value-type
template<class InputIterator>
  using iter-value-type =
    typename std::iterator_traits<InputIterator>::value_type; // exposition only
iter-key-type
template<class InputIterator>
  using iter-key-type = std::remove_const_t<
    std::tuple_element_t<0, iter-value-type<InputIterator>>>; // exposition only
iter-mapped-type
template<class InputIterator>
  using iter-mapped-type =
    std::tuple_element_t<1, iter-value-type<InputIterator>>;  // exposition only
iter-to-alloc-type
template<class InputIterator>
  using iter-to-alloc-type = std::pair<
    std::add_const_t<std::tuple_element_t<0, iter-value-type<InputIterator>>>,
    std::tuple_element_t<1, iter-value-type<InputIterator>>>; // exposition only

Equality Comparisons

operator==
template<class Key, class T, class Hash, class Pred, class Alloc>
  bool operator==(const unordered_map<Key, T, Hash, Pred, Alloc>& x,
                  const unordered_map<Key, T, Hash, Pred, Alloc>& y);

Return true if x.size() == y.size() and for every element in x, there is an element in y with the same key, with an equal value (using operator== to compare the value types).

Notes:

The behavior of this function was changed to match the C++11 standard in Boost 1.48.

Behavior is undefined if the two containers don’t have equivalent equality predicates.
operator!=
template<class Key, class T, class Hash, class Pred, class Alloc>
  bool operator!=(const unordered_map<Key, T, Hash, Pred, Alloc>& x,
                  const unordered_map<Key, T, Hash, Pred, Alloc>& y);

Return false if x.size() == y.size() and for every element in x, there is an element in y with the same key, with an equal value (using operator== to compare the value types).

Notes:

The behavior of this function was changed to match the C++11 standard in Boost 1.48.

Behavior is undefined if the two containers don’t have equivalent equality predicates.

Swap

template<class Key, class T, class Hash, class Pred, class Alloc>
  void swap(unordered_map<Key, T, Hash, Pred, Alloc>& x,
            unordered_map<Key, T, Hash, Pred, Alloc>& y)
    noexcept(noexcept(x.swap(y)));

Swaps the contents of x and y.

If Allocator::propagate_on_container_swap is declared and Allocator::propagate_on_container_swap::value is true then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.

Effects:

x.swap(y)
Throws:

Doesn’t throw an exception unless it is thrown by the copy constructor or copy assignment operator of key_equal or hasher.
Notes:

The exception specifications aren’t quite the same as the C++11 standard, as the equality predicate and hash function are swapped using their copy constructors.

erase_if

template<class K, class T, class H, class P, class A, class Predicate>
  typename unordered_map<K, T, H, P, A>::size_type
    erase_if(unordered_map<K, T, H, P, A>& c, Predicate pred);

Traverses the container c and removes all elements for which the supplied predicate returns true.

Returns:

The number of erased elements.
Notes:

Equivalent to:

auto original_size = c.size();
for (auto i = c.begin(), last = c.end(); i != last; ) {
  if (pred(*i)) {
    i = c.erase(i);
  } else {
    ++i;
  }
}
return original_size - c.size();

Class template unordered_multimap

boost::unordered_multimap — An unordered associative container that associates keys with another value. The same key can be stored multiple times.

Synopsis

// #include <boost/unordered/unordered_map.hpp>

namespace boost {
  template<class Key,
           class T,
           class Hash = boost::hash<Key>,
           class Pred = std::equal_to<Key>,
           class Allocator = std::allocator<std::pair<const Key, T>>>
  class unordered_multimap {
  public:
    // types
    using key_type             = Key;
    using mapped_type          = T;
    using value_type           = std::pair<const Key, T>;
    using hasher               = Hash;
    using key_equal            = Pred;
    using allocator_type       = Allocator;
    using pointer              = typename boost::allocator_traits<Allocator>::pointer;
    using const_pointer        = typename boost::allocator_traits<Allocator>::const_pointer;
    using reference            = value_type&;
    using const_reference      = const value_type&;
    using size_type            = implementation-defined;
    using difference_type      = implementation-defined;

    using iterator             = implementation-defined;
    using const_iterator       = implementation-defined;
    using local_iterator       = implementation-defined;
    using const_local_iterator = implementation-defined;
    using node_type            = implementation-defined;

    // construct/copy/destroy
    unordered_multimap();
    explicit unordered_multimap(size_type n,
                                const hasher& hf = hasher(),
                                const key_equal& eql = key_equal(),
                                const allocator_type& a = allocator_type());
    template<class InputIterator>
      unordered_multimap(InputIterator f, InputIterator l,
                         size_type n = implementation-defined,
                         const hasher& hf = hasher(),
                         const key_equal& eql = key_equal(),
                         const allocator_type& a = allocator_type());
    unordered_multimap(const unordered_multimap& other);
    unordered_multimap(unordered_multimap&& other);
    template<class InputIterator>
      unordered_multimap(InputIterator f, InputIterator l, const allocator_type& a);
    explicit unordered_multimap(const Allocator& a);
    unordered_multimap(const unordered_multimap& other, const Allocator& a);
    unordered_multimap(unordered_multimap&& other, const Allocator& a);
    unordered_multimap(std::initializer_list<value_type> il,
                       size_type n = implementation-defined,
                       const hasher& hf = hasher(),
                       const key_equal& eql = key_equal(),
                       const allocator_type& a = allocator_type());
    unordered_multimap(size_type n, const allocator_type& a);
    unordered_multimap(size_type n, const hasher& hf, const allocator_type& a);
    template<class InputIterator>
      unordered_multimap(InputIterator f, InputIterator l, size_type n, const allocator_type& a);
    template<class InputIterator>
      unordered_multimap(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                         const allocator_type& a);
    unordered_multimap(std::initializer_list<value_type> il, const allocator_type& a);
    unordered_multimap(std::initializer_list<value_type> il, size_type n,
                       const allocator_type& a);
    unordered_multimap(std::initializer_list<value_type> il, size_type n, const hasher& hf,
                       const allocator_type& a);
    ~unordered_multimap();
    unordered_multimap& operator=(const unordered_multimap& other);
    unordered_multimap& operator=(unordered_multimap&& other)
      noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
               boost::is_nothrow_move_assignable_v<Hash> &&
               boost::is_nothrow_move_assignable_v<Pred>);
    unordered_multimap& operator=(std::initializer_list<value_type> il);
    allocator_type get_allocator() const noexcept;

    // iterators
    iterator       begin() noexcept;
    const_iterator begin() const noexcept;
    iterator       end() noexcept;
    const_iterator end() const noexcept;
    const_iterator cbegin() const noexcept;
    const_iterator cend() const noexcept;

    // capacity
    [[nodiscard]] bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;

    // modifiers
    template<class... Args> iterator emplace(Args&&... args);
    template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
    iterator insert(const value_type& obj);
    iterator insert(value_type&& obj);
    template<class P> iterator insert(P&& obj);
    iterator insert(const_iterator hint, const value_type& obj);
    iterator insert(const_iterator hint, value_type&& obj);
    template<class P> iterator insert(const_iterator hint, P&& obj);
    template<class InputIterator> void insert(InputIterator first, InputIterator last);
    void insert(std::initializer_list<value_type> il);

    node_type extract(const_iterator position);
    node_type extract(const key_type& k);
    template<class K> node_type extract(K&& k);
    iterator insert(node_type&& nh);
    iterator insert(const_iterator hint, node_type&& nh);

    iterator  erase(iterator position);
    iterator  erase(const_iterator position);
    size_type erase(const key_type& k);
    template<class K> size_type erase(K&& k);
    iterator  erase(const_iterator first, const_iterator last);
    void      quick_erase(const_iterator position);
    void      erase_return_void(const_iterator position);
    void      swap(unordered_multimap& other)
      noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
               boost::is_nothrow_swappable_v<Hash> &&
               boost::is_nothrow_swappable_v<Pred>);
    void      clear() noexcept;

    template<class H2, class P2>
      void merge(unordered_multimap<Key, T, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_multimap<Key, T, H2, P2, Allocator>&& source);
    template<class H2, class P2>
      void merge(unordered_map<Key, T, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_map<Key, T, H2, P2, Allocator>&& source);

    // observers
    hasher hash_function() const;
    key_equal key_eq() const;

    // map operations
    iterator         find(const key_type& k);
    const_iterator   find(const key_type& k) const;
    template<class K>
      iterator       find(const K& k);
    template<class K>
      const_iterator find(const K& k) const;
    template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
      iterator       find(CompatibleKey const& k, CompatibleHash const& hash,
                          CompatiblePredicate const& eq);
    template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
      const_iterator find(CompatibleKey const& k, CompatibleHash const& hash,
                          CompatiblePredicate const& eq) const;
    size_type        count(const key_type& k) const;
    template<class K>
      size_type      count(const K& k) const;
    bool             contains(const key_type& k) const;
    template<class K>
      bool           contains(const K& k) const;
    std::pair<iterator, iterator>               equal_range(const key_type& k);
    std::pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
    template<class K>
      std::pair<iterator, iterator>             equal_range(const K& k);
    template<class K>
      std::pair<const_iterator, const_iterator> equal_range(const K& k) const;

    // bucket interface
    size_type bucket_count() const noexcept;
    size_type max_bucket_count() const noexcept;
    size_type bucket_size(size_type n) const;
    size_type bucket(const key_type& k) const;
    template<class K> size_type bucket(const K& k) const;
    local_iterator begin(size_type n);
    const_local_iterator begin(size_type n) const;
    local_iterator end(size_type n);
    const_local_iterator end(size_type n) const;
    const_local_iterator cbegin(size_type n) const;
    const_local_iterator cend(size_type n) const;

    // hash policy
    float load_factor() const noexcept;
    float max_load_factor() const noexcept;
    void max_load_factor(float z);
    void rehash(size_type n);
    void reserve(size_type n);
  };

  // Deduction Guides
  template<class InputIterator,
           class Hash = boost::hash<iter-key-type<InputIterator>>,
           class Pred = std::equal_to<iter-key-type<InputIterator>>,
           class Allocator = std::allocator<iter-to-alloc-type<InputIterator>>>
    unordered_multimap(InputIterator, InputIterator, typename see below::size_type = see below,
                       Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_multimap<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Hash,
                            Pred, Allocator>;

  template<class Key, class T, class Hash = boost::hash<Key>,
           class Pred = std::equal_to<Key>,
           class Allocator = std::allocator<std::pair<const Key, T>>>
    unordered_multimap(std::initializer_list<std::pair<Key, T>>,
                       typename see below::size_type = see below, Hash = Hash(),
                       Pred = Pred(), Allocator = Allocator())
      -> unordered_multimap<Key, T, Hash, Pred, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_multimap(InputIterator, InputIterator, typename see below::size_type, Allocator)
      -> unordered_multimap<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
                            boost::hash<iter-key-type<InputIterator>>,
                            std::equal_to<iter-key-type<InputIterator>>, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_multimap(InputIterator, InputIterator, Allocator)
      -> unordered_multimap<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
                            boost::hash<iter-key-type<InputIterator>>,
                            std::equal_to<iter-key-type<InputIterator>>, Allocator>;

  template<class InputIterator, class Hash, class Allocator>
    unordered_multimap(InputIterator, InputIterator, typename see below::size_type, Hash,
                       Allocator)
      -> unordered_multimap<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Hash,
                            std::equal_to<iter-key-type<InputIterator>>, Allocator>;

  template<class Key, class T, class Allocator>
    unordered_multimap(std::initializer_list<std::pair<Key, T>>, typename see below::size_type,
                       Allocator)
      -> unordered_multimap<Key, T, boost::hash<Key>, std::equal_to<Key>, Allocator>;

  template<class Key, class T, class Allocator>
    unordered_multimap(std::initializer_list<std::pair<Key, T>>, Allocator)
      -> unordered_multimap<Key, T, boost::hash<Key>, std::equal_to<Key>, Allocator>;

  template<class Key, class T, class Hash, class Allocator>
    unordered_multimap(std::initializer_list<std::pair<Key, T>>, typename see below::size_type,
                       Hash, Allocator)
      -> unordered_multimap<Key, T, Hash, std::equal_to<Key>, Allocator>;

  // Equality Comparisons
  template<class Key, class T, class Hash, class Pred, class Alloc>
    bool operator==(const unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
                    const unordered_multimap<Key, T, Hash, Pred, Alloc>& y);

  template<class Key, class T, class Hash, class Pred, class Alloc>
    bool operator!=(const unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
                    const unordered_multimap<Key, T, Hash, Pred, Alloc>& y);

  // swap
  template<class Key, class T, class Hash, class Pred, class Alloc>
    void swap(unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
              unordered_multimap<Key, T, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));

  template<class K, class T, class H, class P, class A, class Predicate>
    typename unordered_multimap<K, T, H, P, A>::size_type
      erase_if(unordered_multimap<K, T, H, P, A>& c, Predicate pred);
}

Description

Template Parameters

Key

Key must be Erasable from the container (i.e. allocator_traits can destroy it).

T

T must be Erasable from the container (i.e. allocator_traits can destroy it).

Hash

A unary function object type that acts a hash function for a Key. It takes a single argument of type Key and returns a value of type std::size_t.

Pred

A binary function object that implements an equivalence relation on values of type Key. A binary function object that induces an equivalence relation on values of type Key. It takes two arguments of type Key and returns a value of type bool.

Allocator

An allocator whose value type is the same as the container’s value type.

The elements are organized into buckets. Keys with the same hash code are stored in the same bucket.

The number of buckets can be automatically increased by a call to insert, or as the result of calling rehash.

Typedefs

typedef typename allocator_type::pointer pointer;

value_type* if allocator_type::pointer is not defined.

typedef typename allocator_type::const_pointer const_pointer;

boost::pointer_to_other<pointer, value_type>::type if allocator_type::const_pointer is not defined.

typedef implementation-defined size_type;

An unsigned integral type.

size_type can represent any non-negative value of difference_type.

typedef implementation-defined difference_type;

A signed integral type.

Is identical to the difference type of iterator and const_iterator.

typedef implementation-defined iterator;

An iterator whose value type is value_type.

The iterator category is at least a forward iterator.

Convertible to const_iterator.

typedef implementation-defined const_iterator;

A constant iterator whose value type is value_type.

The iterator category is at least a forward iterator.

typedef implementation-defined local_iterator;

An iterator with the same value type, difference type and pointer and reference type as iterator.

A local_iterator object can be used to iterate through a single bucket.

typedef implementation-defined const_local_iterator;

A constant iterator with the same value type, difference type and pointer and reference type as const_iterator.

A const_local_iterator object can be used to iterate through a single bucket.

typedef implementation-defined node_type;

See node_handle_map for details.

Constructors

Default Constructor
unordered_multimap();

Constructs an empty container using hasher() as the hash function, key_equal() as the key equality predicate, allocator_type() as the allocator and a maximum load factor of 1.0.

Postconditions:

size() == 0
Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Bucket Count Constructor
explicit unordered_multimap(size_type n,
                            const hasher& hf = hasher(),
                            const key_equal& eql = key_equal(),
                            const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate, a as the allocator and a maximum load factor of 1.0.

Postconditions:

size() == 0
Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Iterator Range Constructor
template<class InputIterator>
unordered_multimap(InputIterator f, InputIterator l,
                   size_type n = implementation-defined,
                   const hasher& hf = hasher(),
                   const key_equal& eql = key_equal(),
                   const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate, a as the allocator and a maximum load factor of 1.0 and inserts the elements from [f, l) into it.

Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Copy Constructor
unordered_multimap(const unordered_multimap& other);

The copy constructor. Copies the contained elements, hash function, predicate, maximum load factor and allocator.

If Allocator::select_on_container_copy_construction exists and has the right signature, the allocator will be constructed from its result.

Requires:

value_type is copy constructible
Move Constructor
unordered_multimap(unordered_multimap&& other);

The move constructor.

Notes:

This is implemented using Boost.Move.
Requires:

value_type is move-constructible.

On compilers without rvalue reference support the emulation does not support moving without calling boost::move if value_type is not copyable. So, for example, you can’t return the container from a function.
Iterator Range Constructor with Allocator
template<class InputIterator>
  unordered_multimap(InputIterator f, InputIterator l, const allocator_type& a);

Constructs an empty container using a as the allocator, with the default hash function and key equality predicate and a maximum load factor of 1.0 and inserts the elements from [f, l) into it.

Requires:

hasher, key_equal need to be DefaultConstructible.
Allocator Constructor
explicit unordered_multimap(const Allocator& a);

Constructs an empty container, using allocator a.

Copy Constructor with Allocator
unordered_multimap(const unordered_multimap& other, const Allocator& a);

Constructs an container, copying other's contained elements, hash function, predicate, maximum load factor, but using allocator a.

Move Constructor with Allocator
unordered_multimap(unordered_multimap&& other, const Allocator& a);

Construct a container moving other's contained elements, and having the hash function, predicate and maximum load factor, but using allocate a.

Notes:

This is implemented using Boost.Move.
Requires:

value_type is move insertable.
Initializer List Constructor
unordered_multimap(std::initializer_list<value_type> il,
                   size_type n = implementation-defined,
                   const hasher& hf = hasher(),
                   const key_equal& eql = key_equal(),
                   const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate, a as the allocator and a maximum load factor of 1.0 and inserts the elements from il into it.

Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Bucket Count Constructor with Allocator
unordered_multimap(size_type n, const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, the default hash function and key equality predicate, a as the allocator and a maximum load factor of 1.0.

Postconditions:

size() == 0
Requires:

hasher and key_equal need to be DefaultConstructible.
Bucket Count Constructor with Hasher and Allocator
unordered_multimap(size_type n, const hasher& hf, const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, the default key equality predicate, a as the allocator and a maximum load factor of 1.0.

Postconditions:

size() == 0
Requires:

key_equal needs to be DefaultConstructible.
Iterator Range Constructor with Bucket Count and Allocator
template<class InputIterator>
  unordered_multimap(InputIterator f, InputIterator l, size_type n, const allocator_type& a);

Constructs an empty container with at least n buckets, using a as the allocator, with the default hash function and key equality predicate and a maximum load factor of 1.0 and inserts the elements from [f, l) into it.

Requires:

hasher, key_equal need to be DefaultConstructible.
Iterator Range Constructor with Bucket Count and Hasher
template<class InputIterator>
  unordered_multimap(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                     const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, a as the allocator, with the default key equality predicate and a maximum load factor of 1.0 and inserts the elements from [f, l) into it.

Requires:

key_equal needs to be DefaultConstructible.
initializer_list Constructor with Allocator
unordered_multimap(std::initializer_list<value_type> il, const allocator_type& a);

Constructs an empty container using a as the allocator and a maximum load factor of 1.0 and inserts the elements from il into it.

Requires:

hasher and key_equal need to be DefaultConstructible.
initializer_list Constructor with Bucket Count and Allocator
unordered_multimap(std::initializer_list<value_type> il, size_type n, const allocator_type& a);

Constructs an empty container with at least n buckets, using a as the allocator and a maximum load factor of 1.0 and inserts the elements from il into it.

Requires:

hasher and key_equal need to be DefaultConstructible.
initializer_list Constructor with Bucket Count and Hasher and Allocator
unordered_multimap(std::initializer_list<value_type> il, size_type n, const hasher& hf,
                   const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, a as the allocator and a maximum load factor of 1.0 and inserts the elements from il into it.

Requires:

key_equal needs to be DefaultConstructible.

Destructor

~unordered_multimap();
Note:

The destructor is applied to every element, and all memory is deallocated

Assignment

Copy Assignment
unordered_multimap& operator=(const unordered_multimap& other);

The assignment operator. Copies the contained elements, hash function, predicate and maximum load factor but not the allocator.

If Alloc::propagate_on_container_copy_assignment exists and Alloc::propagate_on_container_copy_assignment::value is true, the allocator is overwritten, if not the copied elements are created using the existing allocator.

Requires:

value_type is copy constructible
Move Assignment
unordered_multimap& operator=(unordered_multimap&& other)
  noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
           boost::is_nothrow_move_assignable_v<Hash> &&
           boost::is_nothrow_move_assignable_v<Pred>);

The move assignment operator.

If Alloc::propagate_on_container_move_assignment exists and Alloc::propagate_on_container_move_assignment::value is true, the allocator is overwritten, if not the moved elements are created using the existing allocator.

Notes:

On compilers without rvalue references, this is emulated using Boost.Move. Note that on some compilers the copy assignment operator may be used in some circumstances.
Requires:

value_type is move constructible.
Initializer List Assignment
unordered_multimap& operator=(std::initializer_list<value_type> il);

Assign from values in initializer list. All existing elements are either overwritten by the new elements or destroyed.

Requires:

value_type is CopyInsertable into the container and CopyAssignable.

Iterators

begin
iterator begin() noexcept;
const_iterator begin() const noexcept;
Returns:

An iterator referring to the first element of the container, or if the container is empty the past-the-end value for the container.
end
iterator       end() noexcept;
const_iterator end() const noexcept;
Returns:

An iterator which refers to the past-the-end value for the container.
cbegin
const_iterator cbegin() const noexcept;
Returns:

A const_iterator referring to the first element of the container, or if the container is empty the past-the-end value for the container.
cend
const_iterator cend() const noexcept;
Returns:

A const_iterator which refers to the past-the-end value for the container.

Size and Capacity

empty
[[nodiscard]] bool empty() const noexcept;
Returns:

size() == 0
size
size_type size() const noexcept;
Returns:

std::distance(begin(), end())
max_size
size_type max_size() const noexcept;
Returns:

size() of the largest possible container.

Modifiers

emplace
template<class... Args> iterator emplace(Args&&... args);

Inserts an object, constructed with the arguments args, in the container.

Requires:

value_type is EmplaceConstructible into X from args.
Returns:

An iterator pointing to the inserted element.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

If the compiler doesn’t support variadic template arguments or rvalue references, this is emulated for up to 10 arguments, with no support for rvalue references or move semantics.

Since existing std::pair implementations don’t support std::piecewise_construct this emulates it, but using boost::unordered::piecewise_construct.
emplace_hint
template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);

Inserts an object, constructed with the arguments args, in the container.

position is a suggestion to where the element should be inserted.

Requires:

value_type is EmplaceConstructible into X from args.
Returns:

An iterator pointing to the inserted element.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

If the compiler doesn’t support variadic template arguments or rvalue references, this is emulated for up to 10 arguments, with no support for rvalue references or move semantics.

Since existing std::pair implementations don’t support std::piecewise_construct this emulates it, but using boost::unordered::piecewise_construct.
Copy Insert
iterator insert(const value_type& obj);

Inserts obj in the container.

Requires:

value_type is CopyInsertable.
Returns:

An iterator pointing to the inserted element.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Move Insert
iterator insert(value_type&& obj);

Inserts obj in the container.

Requires:

value_type is MoveInsertable.
Returns:

An iterator pointing to the inserted element.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Emplace Insert
template<class P> iterator insert(P&& obj);

Inserts an element into the container by performing emplace(std::forward<P>(value)).

Only participates in overload resolution if std::is_constructible<value_type, P&&>::value is true.

Returns:

An iterator pointing to the inserted element.
Copy Insert with Hint
iterator insert(const_iterator hint, const value_type& obj);

Inserts obj in the container.

hint is a suggestion to where the element should be inserted.

Requires:

value_type is CopyInsertable.
Returns:

An iterator pointing to the inserted element.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Move Insert with Hint
iterator insert(const_iterator hint, value_type&& obj);

Inserts obj in the container.

hint is a suggestion to where the element should be inserted.

Requires:

value_type is MoveInsertable.
Returns:

An iterator pointing to the inserted element.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Emplace Insert with Hint
template<class P> iterator insert(const_iterator hint, P&& obj);

Inserts an element into the container by performing emplace_hint(hint, std::forward<P>(value)).

Only participates in overload resolution if std::is_constructible<value_type, P&&>::value is true.

hint is a suggestion to where the element should be inserted.

Returns:

An iterator pointing to the inserted element.
Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Insert Iterator Range
template<class InputIterator> void insert(InputIterator first, InputIterator last);

Inserts a range of elements into the container.

Requires:

value_type is EmplaceConstructible into X from *first.
Throws:

When inserting a single element, if an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Insert Initializer List
void insert(std::initializer_list<value_type> il);

Inserts a range of elements into the container.

Requires:

value_type is EmplaceConstructible into X from *first.
Throws:

When inserting a single element, if an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Extract by Iterator
node_type extract(const_iterator position);

Removes the element pointed to by position.

Returns:

A node_type owning the element.
Notes:

A node extracted using this method can be inserted into a compatible unordered_map.
Extract by Key
node_type extract(const key_type& k);
template<class K> node_type extract(K&& k);

Removes an element with key equivalent to k.

Returns:

A node_type owning the element if found, otherwise an empty node_type.
Throws:

Only throws an exception if it is thrown by hasher or key_equal.
Notes:

A node extracted using this method can be inserted into a compatible unordered_map.

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
Insert with node_handle
iterator insert(node_type&& nh);

If nh is empty, has no effect.

Otherwise inserts the element owned by nh.

Requires:

nh is empty or nh.get_allocator() is equal to the container’s allocator.
Returns:

If nh was empty, returns end().

Otherwise returns an iterator pointing to the newly inserted element.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

This can be used to insert a node extracted from a compatible unordered_map.
Insert with Hint and node_handle
iterator insert(const_iterator hint, node_type&& nh);

If nh is empty, has no effect.

Otherwise inserts the element owned by nh.

hint is a suggestion to where the element should be inserted.

Requires:

nh is empty or nh.get_allocator() is equal to the container’s allocator.
Returns:

If nh was empty, returns end().

Otherwise returns an iterator pointing to the newly inserted element.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

This can be used to insert a node extracted from a compatible unordered_map.
Erase by Position
iterator erase(iterator position);
iterator erase(const_iterator position);

Erase the element pointed to by position.

Returns:

The iterator following position before the erasure.
Throws:

Only throws an exception if it is thrown by hasher or key_equal.
Notes:

In older versions this could be inefficient because it had to search through several buckets to find the position of the returned iterator. The data structure has been changed so that this is no longer the case, and the alternative erase methods have been deprecated.
Erase by Key
size_type erase(const key_type& k);
template<class K> size_type erase(K&& k);

Erase all elements with key equivalent to k.

Returns:

The number of elements erased.
Throws:

Only throws an exception if it is thrown by hasher or key_equal.
Notes:

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
Erase Range
iterator erase(const_iterator first, const_iterator last);

Erases the elements in the range from first to last.

Returns:

The iterator following the erased elements - i.e. last.
Throws:

Only throws an exception if it is thrown by hasher or key_equal.

In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations.
quick_erase
void quick_erase(const_iterator position);

Erase the element pointed to by position.

Throws:

Only throws an exception if it is thrown by hasher or key_equal.

In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations.
Notes:

This method was implemented because returning an iterator to the next element from erase was expensive, but the container has been redesigned so that is no longer the case. So this method is now deprecated.
erase_return_void
void erase_return_void(const_iterator position);

Erase the element pointed to by position.

Throws:

Only throws an exception if it is thrown by hasher or key_equal.

In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations.
Notes:

This method was implemented because returning an iterator to the next element from erase was expensive, but the container has been redesigned so that is no longer the case. So this method is now deprecated.
swap
void swap(unordered_multimap& other)
  noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
           boost::is_nothrow_swappable_v<Hash> &&
           boost::is_nothrow_swappable_v<Pred>);

Swaps the contents of the container with the parameter.

If Allocator::propagate_on_container_swap is declared and Allocator::propagate_on_container_swap::value is true then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.

Throws:

Doesn’t throw an exception unless it is thrown by the copy constructor or copy assignment operator of key_equal or hasher.
Notes:

The exception specifications aren’t quite the same as the C++11 standard, as the equality predicate and hash function are swapped using their copy constructors.
clear
void clear() noexcept;

Erases all elements in the container.

Postconditions:

size() == 0
Throws:

Never throws an exception.
merge
template<class H2, class P2>
  void merge(unordered_multimap<Key, T, H2, P2, Allocator>& source);
template<class H2, class P2>
  void merge(unordered_multimap<Key, T, H2, P2, Allocator>&& source);
template<class H2, class P2>
  void merge(unordered_map<Key, T, H2, P2, Allocator>& source);
template<class H2, class P2>
  void merge(unordered_map<Key, T, H2, P2, Allocator>&& source);

Attempt to "merge" two containers by iterating source and extracting all nodes in source and inserting them into *this.

Because source can have a different hash function and key equality predicate, the key of each node in source is rehashed using this->hash_function() and then, if required, compared using this->key_eq().

The behavior of this function is undefined if this->get_allocator() != source.get_allocator().

This function does not copy or move any elements and instead simply relocates the nodes from source into *this.

Notes:

  • Pointers and references to transferred elements remain valid.

  • Invalidates iterators to transferred elements.

  • Invalidates iterators belonging to *this.

  • Iterators to non-transferred elements in source remain valid.

Observers

get_allocator
allocator_type get_allocator() const;
hash_function
hasher hash_function() const;
Returns:

The container’s hash function.
key_eq
key_equal key_eq() const;
Returns:

The container’s key equality predicate

Lookup

find
iterator         find(const key_type& k);
const_iterator   find(const key_type& k) const;
template<class K>
  iterator       find(const K& k);
template<class K>
  const_iterator find(const K& k) const;
template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
  iterator       find(CompatibleKey const& k, CompatibleHash const& hash,
                      CompatiblePredicate const& eq);
template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
  const_iterator find(CompatibleKey const& k, CompatibleHash const& hash,
                      CompatiblePredicate const& eq) const;
Returns:

An iterator pointing to an element with key equivalent to k, or b.end() if no such element exists.
Notes:

The templated overloads containing CompatibleKey, CompatibleHash and CompatiblePredicate are non-standard extensions which allow you to use a compatible hash function and equality predicate for a key of a different type in order to avoid an expensive type cast. In general, its use is not encouraged and instead the K member function templates should be used.

The template <typename K> overloads only participate in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
count
size_type        count(const key_type& k) const;
template<class K>
  size_type      count(const K& k) const;
Returns:

The number of elements with key equivalent to k.
Notes:

The template <typename K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
contains
bool             contains(const key_type& k) const;
template<class K>
  bool           contains(const K& k) const;
Returns:

A boolean indicating whether or not there is an element with key equal to key in the container
Notes:

The template <typename K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
equal_range
std::pair<iterator, iterator>               equal_range(const key_type& k);
std::pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
template<class K>
  std::pair<iterator, iterator>             equal_range(const K& k);
template<class K>
  std::pair<const_iterator, const_iterator> equal_range(const K& k) const;
Returns:

A range containing all elements with key equivalent to k. If the container doesn’t contain any such elements, returns std::make_pair(b.end(), b.end()).
Notes:

The template <typename K> overloads only participate in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.

Bucket Interface

bucket_count
size_type bucket_count() const noexcept;
Returns:

The number of buckets.
max_bucket_count
size_type max_bucket_count() const noexcept;
Returns:

An upper bound on the number of buckets.
bucket_size
size_type bucket_size(size_type n) const;
Requires:

n < bucket_count()
Returns:

The number of elements in bucket n.
bucket
size_type bucket(const key_type& k) const;
template<class K> size_type bucket(const K& k) const;
Returns:

The index of the bucket which would contain an element with key k.
Postconditions:

The return value is less than bucket_count().
Notes:

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
begin
local_iterator begin(size_type n);
const_local_iterator begin(size_type n) const;
Requires:

n shall be in the range [0, bucket_count()).
Returns:

A local iterator pointing the first element in the bucket with index n.
end
local_iterator end(size_type n);
const_local_iterator end(size_type n) const;
Requires:

n shall be in the range [0, bucket_count()).
Returns:

A local iterator pointing the 'one past the end' element in the bucket with index n.
cbegin
const_local_iterator cbegin(size_type n) const;
Requires:

n shall be in the range [0, bucket_count()).
Returns:

A constant local iterator pointing the first element in the bucket with index n.
cend
const_local_iterator cend(size_type n) const;
Requires:

n shall be in the range [0, bucket_count()).
Returns:

A constant local iterator pointing the 'one past the end' element in the bucket with index n.

Hash Policy

load_factor
float load_factor() const noexcept;
Returns:

The average number of elements per bucket.
max_load_factor
float max_load_factor() const noexcept;
Returns:

Returns the current maximum load factor.
Set max_load_factor
void max_load_factor(float z);
Effects:

Changes the container’s maximum load factor, using z as a hint.
rehash
void rehash(size_type n);

Changes the number of buckets so that there are at least n buckets, and so that the load factor is less than or equal to the maximum load factor. When applicable, this will either grow or shrink the bucket_count() associated with the container.

When size() == 0, rehash(0) will deallocate the underlying buckets array.

Invalidates iterators, and changes the order of elements. Pointers and references to elements are not invalidated.

Throws:

The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function.
reserve
void reserve(size_type n);

Equivalent to a.rehash(ceil(n / a.max_load_factor())), or a.rehash(1) if n > 0 and a.max_load_factor() == std::numeric_limits<float>::infinity().

Similar to rehash, this function can be used to grow or shrink the number of buckets in the container.

Invalidates iterators, and changes the order of elements. Pointers and references to elements are not invalidated.

Throws:

The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function.

Deduction Guides

A deduction guide will not participate in overload resolution if any of the following are true:

  • It has an InputIterator template parameter and a type that does not qualify as an input iterator is deduced for that parameter.

  • It has an Allocator template parameter and a type that does not qualify as an allocator is deduced for that parameter.

  • It has a Hash template parameter and an integral type or a type that qualifies as an allocator is deduced for that parameter.

  • It has a Pred template parameter and a type that qualifies as an allocator is deduced for that parameter.

A size_­type parameter type in a deduction guide refers to the size_­type member type of the container type deduced by the deduction guide. Its default value coincides with the default value of the constructor selected.

iter-value-type
template<class InputIterator>
  using iter-value-type =
    typename std::iterator_traits<InputIterator>::value_type; // exposition only
iter-key-type
template<class InputIterator>
  using iter-key-type = std::remove_const_t<
    std::tuple_element_t<0, iter-value-type<InputIterator>>>; // exposition only
iter-mapped-type
template<class InputIterator>
  using iter-mapped-type =
    std::tuple_element_t<1, iter-value-type<InputIterator>>;  // exposition only
iter-to-alloc-type
template<class InputIterator>
  using iter-to-alloc-type = std::pair<
    std::add_const_t<std::tuple_element_t<0, iter-value-type<InputIterator>>>,
    std::tuple_element_t<1, iter-value-type<InputIterator>>>; // exposition only

Equality Comparisons

operator==
template<class Key, class T, class Hash, class Pred, class Alloc>
  bool operator==(const unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
                  const unordered_multimap<Key, T, Hash, Pred, Alloc>& y);

Return true if x.size() == y.size() and for every equivalent key group in x, there is a group in y for the same key, which is a permutation (using operator== to compare the value types).

Notes:

The behavior of this function was changed to match the C++11 standard in Boost 1.48.

Behavior is undefined if the two containers don’t have equivalent equality predicates.
operator!=
template<class Key, class T, class Hash, class Pred, class Alloc>
  bool operator!=(const unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
                  const unordered_multimap<Key, T, Hash, Pred, Alloc>& y);

Return false if x.size() == y.size() and for every equivalent key group in x, there is a group in y for the same key, which is a permutation (using operator== to compare the value types).

Notes:

The behavior of this function was changed to match the C++11 standard in Boost 1.48.

Behavior is undefined if the two containers don’t have equivalent equality predicates.

Swap

template<class Key, class T, class Hash, class Pred, class Alloc>
  void swap(unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
            unordered_multimap<Key, T, Hash, Pred, Alloc>& y)
    noexcept(noexcept(x.swap(y)));

Swaps the contents of x and y.

If Allocator::propagate_on_container_swap is declared and Allocator::propagate_on_container_swap::value is true then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.

Effects:

x.swap(y)
Throws:

Doesn’t throw an exception unless it is thrown by the copy constructor or copy assignment operator of key_equal or hasher.
Notes:

The exception specifications aren’t quite the same as the C++11 standard, as the equality predicate and hash function are swapped using their copy constructors.

erase_if

template<class K, class T, class H, class P, class A, class Predicate>
  typename unordered_multimap<K, T, H, P, A>::size_type
    erase_if(unordered_multimap<K, T, H, P, A>& c, Predicate pred);

Traverses the container c and removes all elements for which the supplied predicate returns true.

Returns:

The number of erased elements.
Notes:

Equivalent to:

auto original_size = c.size();
for (auto i = c.begin(), last = c.end(); i != last; ) {
  if (pred(*i)) {
    i = c.erase(i);
  } else {
    ++i;
  }
}
return original_size - c.size();

Class template unordered_set

boost::unordered_set — An unordered associative container that stores unique values.

Synopsis

// #include <boost/unordered/unordered_set.hpp>

namespace boost {
  template<class Key,
           class Hash = boost::hash<Key>,
           class Pred = std::equal_to<Key>,
           class Allocator = std::allocator<Key>>
  class unordered_set {
  public:
    // types
    using key_type             = Key;
    using value_type           = Key;
    using hasher               = Hash;
    using key_equal            = Pred;
    using allocator_type       = Allocator;
    using pointer              = typename boost::allocator_traits<Allocator>::pointer;
    using const_pointer        = typename boost::allocator_traits<Allocator>::const_pointer;
    using reference            = value_type&;
    using const_reference      = const value_type&;
    using size_type            = implementation-defined;
    using difference_type      = implementation-defined;

    using iterator             = implementation-defined;
    using const_iterator       = implementation-defined;
    using local_iterator       = implementation-defined;
    using const_local_iterator = implementation-defined;
    using node_type            = implementation-defined;
    using insert_return_type   = implementation-defined;

    // construct/copy/destroy
    unordered_set();
    explicit unordered_set(size_type n,
                           const hasher& hf = hasher(),
                           const key_equal& eql = key_equal(),
                           const allocator_type& a = allocator_type());
    template<class InputIterator>
      unordered_set(InputIterator f, InputIterator l,
                    size_type n = implementation-defined,
                    const hasher& hf = hasher(),
                    const key_equal& eql = key_equal(),
                    const allocator_type& a = allocator_type());
    unordered_set(const unordered_set& other);
    unordered_set(unordered_set&& other);
    template<class InputIterator>
      unordered_set(InputIterator f, InputIterator l, const allocator_type& a);
    explicit unordered_set(const Allocator& a);
    unordered_set(const unordered_set& other, const Allocator& a);
    unordered_set(unordered_set&& other, const Allocator& a);
    unordered_set(std::initializer_list<value_type> il,
                  size_type n = implementation-defined,
                  const hasher& hf = hasher(),
                  const key_equal& eql = key_equal(),
                  const allocator_type& a = allocator_type());
    unordered_set(size_type n, const allocator_type& a);
    unordered_set(size_type n, const hasher& hf, const allocator_type& a);
    template<class InputIterator>
      unordered_set(InputIterator f, InputIterator l, size_type n, const allocator_type& a);
    template<class InputIterator>
      unordered_set(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                    const allocator_type& a);
    unordered_set(std::initializer_list<value_type> il, const allocator_type& a);
    unordered_set(std::initializer_list<value_type> il, size_type n, const allocator_type& a);
    unordered_set(std::initializer_list<value_type> il, size_type n, const hasher& hf,
                  const allocator_type& a);
    ~unordered_set();
    unordered_set& operator=(const unordered_set& other);
    unordered_set& operator=(unordered_set&& other)
      noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
               boost::is_nothrow_move_assignable_v<Hash> &&
               boost::is_nothrow_move_assignable_v<Pred>);
    unordered_set& operator=(std::initializer_list<value_type> il);
    allocator_type get_allocator() const noexcept;

    // iterators
    iterator       begin() noexcept;
    const_iterator begin() const noexcept;
    iterator       end() noexcept;
    const_iterator end() const noexcept;
    const_iterator cbegin() const noexcept;
    const_iterator cend() const noexcept;

    // capacity
    [[nodiscard]] bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;

    // modifiers
    template<class... Args> std::pair<iterator, bool> emplace(Args&&... args);
    template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
    std::pair<iterator, bool> insert(const value_type& obj);
    std::pair<iterator, bool> insert(value_type&& obj);
    template<class K> std::pair<iterator, bool> insert(K&& k);
    iterator insert(const_iterator hint, const value_type& obj);
    iterator insert(const_iterator hint, value_type&& obj);
    template<class K> iterator insert(const_iterator hint, K&& k);
    template<class InputIterator> void insert(InputIterator first, InputIterator last);
    void insert(std::initializer_list<value_type>);

    node_type extract(const_iterator position);
    node_type extract(const key_type& k);
    template<class K> node_type extract(K&& k);
    insert_return_type insert(node_type&& nh);
    iterator           insert(const_iterator hint, node_type&& nh);

    iterator  erase(iterator position);
    iterator  erase(const_iterator position);
    size_type erase(const key_type& k);
    template<class K> size_type erase(K&& k);
    iterator  erase(const_iterator first, const_iterator last);
    void      quick_erase(const_iterator position);
    void      erase_return_void(const_iterator position);
    void      swap(unordered_set& other)
      noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
               boost::is_nothrow_swappable_v<Hash> &&
               boost::is_nothrow_swappable_v<Pred>);
    void      clear() noexcept;

    template<class H2, class P2>
      void merge(unordered_set<Key, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_set<Key, H2, P2, Allocator>&& source);
    template<class H2, class P2>
      void merge(unordered_multiset<Key, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_multiset<Key, H2, P2, Allocator>&& source);

    // observers
    hasher hash_function() const;
    key_equal key_eq() const;

    // set operations
    iterator         find(const key_type& k);
    const_iterator   find(const key_type& k) const;
    template<class K>
      iterator       find(const K& k);
    template<class K>
      const_iterator find(const K& k) const;
    template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
      iterator       find(CompatibleKey const& k, CompatibleHash const& hash,
                          CompatiblePredicate const& eq);
    template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
      const_iterator find(CompatibleKey const& k, CompatibleHash const& hash,
                          CompatiblePredicate const& eq) const;
    size_type        count(const key_type& k) const;
    template<class K>
      size_type      count(const K& k) const;
    bool             contains(const key_type& k) const;
    template<class K>
      bool           contains(const K& k) const;
    std::pair<iterator, iterator>               equal_range(const key_type& k);
    std::pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
    template<class K>
      std::pair<iterator, iterator>             equal_range(const K& k);
    template<class K>
      std::pair<const_iterator, const_iterator> equal_range(const K& k) const;

    // bucket interface
    size_type bucket_count() const noexcept;
    size_type max_bucket_count() const noexcept;
    size_type bucket_size(size_type n) const;
    size_type bucket(const key_type& k) const;
    template<class K> size_type bucket(const K& k) const;
    local_iterator begin(size_type n);
    const_local_iterator begin(size_type n) const;
    local_iterator end(size_type n);
    const_local_iterator end(size_type n) const;
    const_local_iterator cbegin(size_type n) const;
    const_local_iterator cend(size_type n) const;

    // hash policy
    float load_factor() const noexcept;
    float max_load_factor() const noexcept;
    void max_load_factor(float z);
    void rehash(size_type n);
    void reserve(size_type n);
  };

  // Deduction Guides
  template<class InputIterator,
           class Hash = boost::hash<iter-value-type<InputIterator>>,
           class Pred = std::equal_to<iter-value-type<InputIterator>>,
           class Allocator = std::allocator<iter-value-type<InputIterator>>>
    unordered_set(InputIterator, InputIterator, typename see below::size_type = see below,
                  Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_set<iter-value-type<InputIterator>, Hash, Pred, Allocator>;

  template<class T, class Hash = boost::hash<T>, class Pred = std::equal_to<T>,
           class Allocator = std::allocator<T>>
    unordered_set(std::initializer_list<T>, typename see below::size_type = see below,
                  Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_set<T, Hash, Pred, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_set(InputIterator, InputIterator, typename see below::size_type, Allocator)
      -> unordered_set<iter-value-type<InputIterator>,
                       boost::hash<iter-value-type<InputIterator>>,
                       std::equal_to<iter-value-type<InputIterator>>, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_set(InputIterator, InputIterator, Allocator)
      -> unordered_set<iter-value-type<InputIterator>,
                       boost::hash<iter-value-type<InputIterator>>,
                       std::equal_to<iter-value-type<InputIterator>>, Allocator>;

  template<class InputIterator, class Hash, class Allocator>
    unordered_set(InputIterator, InputIterator, typename see below::size_type, Hash, Allocator)
      -> unordered_set<iter-value-type<InputIterator>, Hash,
                       std::equal_to<iter-value-type<InputIterator>>, Allocator>;

  template<class T, class Allocator>
    unordered_set(std::initializer_list<T>, typename see below::size_type, Allocator)
      -> unordered_set<T, boost::hash<T>, std::equal_to<T>, Allocator>;

  template<class T, class Allocator>
    unordered_set(std::initializer_list<T>, Allocator)
      -> unordered_set<T, boost::hash<T>, std::equal_to<T>, Allocator>;

  template<class T, class Hash, class Allocator>
    unordered_set(std::initializer_list<T>, typename see below::size_type, Hash, Allocator)
      -> unordered_set<T, Hash, std::equal_to<T>, Allocator>;

  // Equality Comparisons
  template<class Key, class Hash, class Pred, class Alloc>
    bool operator==(const unordered_set<Key, Hash, Pred, Alloc>& x,
                    const unordered_set<Key, Hash, Pred, Alloc>& y);

  template<class Key, class Hash, class Pred, class Alloc>
    bool operator!=(const unordered_set<Key, Hash, Pred, Alloc>& x,
                    const unordered_set<Key, Hash, Pred, Alloc>& y);

  // swap
  template<class Key, class Hash, class Pred, class Alloc>
    void swap(unordered_set<Key, Hash, Pred, Alloc>& x,
              unordered_set<Key, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));

  template<class K, class H, class P, class A, class Predicate>
    typename unordered_set<K, H, P, A>::size_type
      erase_if(unordered_set<K, H, P, A>& c, Predicate pred);
}

Description

Template Parameters

Key

Key must be Erasable from the container (i.e. allocator_traits can destroy it).

Hash

A unary function object type that acts a hash function for a Key. It takes a single argument of type Key and returns a value of type std::size_t.

Pred

A binary function object that implements an equivalence relation on values of type Key. A binary function object that induces an equivalence relation on values of type Key. It takes two arguments of type Key and returns a value of type bool.

Allocator

An allocator whose value type is the same as the container’s value type.

The elements are organized into buckets. Keys with the same hash code are stored in the same bucket.

The number of buckets can be automatically increased by a call to insert, or as the result of calling rehash.

Typedefs

typedef typename allocator_type::pointer pointer;

value_type* if allocator_type::pointer is not defined.

typedef typename allocator_type::const_pointer const_pointer;

boost::pointer_to_other<pointer, value_type>::type if allocator_type::const_pointer is not defined.

typedef implementation-defined size_type;

An unsigned integral type.

size_type can represent any non-negative value of difference_type.

typedef implementation-defined difference_type;

A signed integral type.

Is identical to the difference type of iterator and const_iterator.

typedef implementation-defined iterator;

A constant iterator whose value type is value_type.

The iterator category is at least a forward iterator.

Convertible to const_iterator.

typedef implementation-defined const_iterator;

A constant iterator whose value type is value_type.

The iterator category is at least a forward iterator.

typedef implementation-defined local_iterator;

An iterator with the same value type, difference type and pointer and reference type as iterator.

A local_iterator object can be used to iterate through a single bucket.

typedef implementation-defined const_local_iterator;

A constant iterator with the same value type, difference type and pointer and reference type as const_iterator.

A const_local_iterator object can be used to iterate through a single bucket.

typedef implementation-defined node_type;

See node_handle_set for details.

typedef implementation-defined insert_return_type;

Structure returned by inserting node_type.

Constructors

Default Constructor
unordered_set();

Constructs an empty container using hasher() as the hash function, key_equal() as the key equality predicate, allocator_type() as the allocator and a maximum load factor of 1.0.

Postconditions:

size() == 0
Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Bucket Count Constructor
explicit unordered_set(size_type n,
                       const hasher& hf = hasher(),
                       const key_equal& eql = key_equal(),
                       const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate, a as the allocator and a maximum load factor of 1.0.

Postconditions:

size() == 0
Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Iterator Range Constructor
template<class InputIterator>
  unordered_set(InputIterator f, InputIterator l,
                size_type n = implementation-defined,
                const hasher& hf = hasher(),
                const key_equal& eql = key_equal(),
                const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate, a as the allocator and a maximum load factor of 1.0 and inserts the elements from [f, l) into it.

Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Copy Constructor
unordered_set(const unordered_set& other);

The copy constructor. Copies the contained elements, hash function, predicate, maximum load factor and allocator.

If Allocator::select_on_container_copy_construction exists and has the right signature, the allocator will be constructed from its result.

Requires:

value_type is copy constructible
Move Constructor
unordered_set(unordered_set&& other);

The move constructor.

Notes:

This is implemented using Boost.Move.
Requires:

value_type is move-constructible.

On compilers without rvalue reference support the emulation does not support moving without calling boost::move if value_type is not copyable. So, for example, you can’t return the container from a function.
Iterator Range Constructor with Allocator
template<class InputIterator>
  unordered_set(InputIterator f, InputIterator l, const allocator_type& a);

Constructs an empty container using a as the allocator, with the default hash function and key equality predicate and a maximum load factor of 1.0 and inserts the elements from [f, l) into it.

Requires:

hasher, key_equal need to be DefaultConstructible.
Allocator Constructor
explicit unordered_set(const Allocator& a);

Constructs an empty container, using allocator a.

Copy Constructor with Allocator
unordered_set(const unordered_set& other, const Allocator& a);

Constructs an container, copying other's contained elements, hash function, predicate, maximum load factor, but using allocator a.

Move Constructor with Allocator
unordered_set(unordered_set&& other, const Allocator& a);

Construct a container moving other's contained elements, and having the hash function, predicate and maximum load factor, but using allocate a.

Notes:

This is implemented using Boost.Move.
Requires:

value_type is move insertable.
Initializer List Constructor
unordered_set(std::initializer_list<value_type> il,
              size_type n = implementation-defined,
              const hasher& hf = hasher(),
              const key_equal& eql = key_equal(),
              const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate, a as the allocator and a maximum load factor of 1.0 and inserts the elements from il into it.

Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Bucket Count Constructor with Allocator
unordered_set(size_type n, const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, the default hash function and key equality predicate, a as the allocator and a maximum load factor of 1.0.

Postconditions:

size() == 0
Requires:

hasher and key_equal need to be DefaultConstructible.
Bucket Count Constructor with Hasher and Allocator
unordered_set(size_type n, const hasher& hf, const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, the default key equality predicate, a as the allocator and a maximum load factor of 1.0.

Postconditions:

size() == 0
Requires:

key_equal needs to be DefaultConstructible.
Iterator Range Constructor with Bucket Count and Allocator
template<class InputIterator>
  unordered_set(InputIterator f, InputIterator l, size_type n, const allocator_type& a);

Constructs an empty container with at least n buckets, using a as the allocator, with the default hash function and key equality predicate and a maximum load factor of 1.0 and inserts the elements from [f, l) into it.

Requires:

hasher, key_equal need to be DefaultConstructible.
Iterator Range Constructor with Bucket Count and Hasher
template<class InputIterator>
  unordered_set(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, a as the allocator, with the default key equality predicate and a maximum load factor of 1.0 and inserts the elements from [f, l) into it.

Requires:

key_equal needs to be DefaultConstructible.
initializer_list Constructor with Allocator
unordered_set(std::initializer_list<value_type> il, const allocator_type& a);

Constructs an empty container using a as the allocator and a maximum load factor of 1.0 and inserts the elements from il into it.

Requires:

hasher and key_equal need to be DefaultConstructible.
initializer_list Constructor with Bucket Count and Allocator
unordered_set(std::initializer_list<value_type> il, size_type n, const allocator_type& a);

Constructs an empty container with at least n buckets, using a as the allocator and a maximum load factor of 1.0 and inserts the elements from il into it.

Requires:

hasher and key_equal need to be DefaultConstructible.
initializer_list Constructor with Bucket Count and Hasher and Allocator
unordered_set(std::initializer_list<value_type> il, size_type n, const hasher& hf,
              const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, a as the allocator and a maximum load factor of 1.0 and inserts the elements from il into it.

Requires:

key_equal needs to be DefaultConstructible.

Destructor

~unordered_set();
Note:

The destructor is applied to every element, and all memory is deallocated

Assignment

Copy Assignment
unordered_set& operator=(const unordered_set& other);

The assignment operator. Copies the contained elements, hash function, predicate and maximum load factor but not the allocator.

If Alloc::propagate_on_container_copy_assignment exists and Alloc::propagate_on_container_copy_assignment::value is true, the allocator is overwritten, if not the copied elements are created using the existing allocator.

Requires:

value_type is copy constructible
Move Assignment
unordered_set& operator=(unordered_set&& other)
  noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
           boost::is_nothrow_move_assignable_v<Hash> &&
           boost::is_nothrow_move_assignable_v<Pred>);

The move assignment operator.

If Alloc::propagate_on_container_move_assignment exists and Alloc::propagate_on_container_move_assignment::value is true, the allocator is overwritten, if not the moved elements are created using the existing allocator.

Notes:

On compilers without rvalue references, this is emulated using Boost.Move. Note that on some compilers the copy assignment operator may be used in some circumstances.
Requires:

value_type is move constructible.
Initializer List Assignment
unordered_set& operator=(std::initializer_list<value_type> il);

Assign from values in initializer list. All existing elements are either overwritten by the new elements or destroyed.

Requires:

value_type is CopyInsertable into the container and CopyAssignable.

Iterators

begin
iterator       begin() noexcept;
const_iterator begin() const noexcept;
Returns:

An iterator referring to the first element of the container, or if the container is empty the past-the-end value for the container.
end
iterator       end() noexcept;
const_iterator end() const noexcept;
Returns:

An iterator which refers to the past-the-end value for the container.
cbegin
const_iterator cbegin() const noexcept;
Returns:

A const_iterator referring to the first element of the container, or if the container is empty the past-the-end value for the container.
cend
const_iterator cend() const noexcept;
Returns:

A const_iterator which refers to the past-the-end value for the container.

Size and Capacity

empty
[[nodiscard]] bool empty() const noexcept;
Returns:

size() == 0
size
size_type size() const noexcept;
Returns:

std::distance(begin(), end())
max_size
size_type max_size() const noexcept;
Returns:

size() of the largest possible container.

Modifiers

emplace
template<class... Args> std::pair<iterator, bool> emplace(Args&&... args);

Inserts an object, constructed with the arguments args, in the container if and only if there is no element in the container with an equivalent value.

Requires:

value_type is EmplaceConstructible into X from args.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent value.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

If the compiler doesn’t support variadic template arguments or rvalue references, this is emulated for up to 10 arguments, with no support for rvalue references or move semantics.

Since existing std::pair implementations don’t support std::piecewise_construct this emulates it, but using boost::unordered::piecewise_construct.
emplace_hint
template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);

Inserts an object, constructed with the arguments args, in the container if and only if there is no element in the container with an equivalent value.

position is a suggestion to where the element should be inserted.

Requires:

value_type is EmplaceConstructible into X from args.
Returns:

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

If the compiler doesn’t support variadic template arguments or rvalue references, this is emulated for up to 10 arguments, with no support for rvalue references or move semantics.

Since existing std::pair implementations don’t support std::piecewise_construct this emulates it, but using boost::unordered::piecewise_construct.
Copy Insert
std::pair<iterator, bool> insert(const value_type& obj);

Inserts obj in the container if and only if there is no element in the container with an equivalent key.

Requires:

value_type is CopyInsertable.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Move Insert
std::pair<iterator, bool> insert(value_type&& obj);

Inserts obj in the container if and only if there is no element in the container with an equivalent key.

Requires:

value_type is MoveInsertable.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Transparent Insert
template<class K> std::pair<iterator, bool> insert(K&& k);

Inserts an element constructed from std::forward<K>(k) in the container if and only if there is no element in the container with an equivalent key.

Requires:

value_type is EmplaceConstructible from k.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

This overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
Copy Insert with Hint
iterator insert(const_iterator hint, const value_type& obj);

Inserts obj in the container if and only if there is no element in the container with an equivalent key.

hint is a suggestion to where the element should be inserted.

Requires:

value_type is CopyInsertable.
Returns:

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Move Insert with Hint
iterator insert(const_iterator hint, value_type&& obj);

Inserts obj in the container if and only if there is no element in the container with an equivalent key.

hint is a suggestion to where the element should be inserted.

Requires:

value_type is MoveInsertable.
Returns:

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Transparent Insert with Hint
template<class K> iterator insert(const_iterator hint, K&& k);

Inserts an element constructed from std::forward<K>(k) in the container if and only if there is no element in the container with an equivalent key.

hint is a suggestion to where the element should be inserted.

Requires:

value_type is EmplaceConstructible from k.
Returns:

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

This overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
Insert Iterator Range
template<class InputIterator> void insert(InputIterator first, InputIterator last);

Inserts a range of elements into the container. Elements are inserted if and only if there is no element in the container with an equivalent key.

Requires:

value_type is EmplaceConstructible into X from *first.
Throws:

When inserting a single element, if an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Insert Initializer List
void insert(std::initializer_list<value_type>);

Inserts a range of elements into the container. Elements are inserted if and only if there is no element in the container with an equivalent key.

Requires:

value_type is EmplaceConstructible into X from *first.
Throws:

When inserting a single element, if an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Extract by Iterator
node_type extract(const_iterator position);

Removes the element pointed to by position.

Returns:

A node_type owning the element.
Notes:

In C++17 a node extracted using this method can be inserted into a compatible unordered_multiset, but that is not supported yet.
Extract by Value
node_type extract(const key_type& k);
template<class K> node_type extract(K&& k);

Removes an element with key equivalent to k.

Returns:

A node_type owning the element if found, otherwise an empty node_type.
Throws:

Only throws an exception if it is thrown by hasher or key_equal.
Notes:

In C++17 a node extracted using this method can be inserted into a compatible unordered_multiset, but that is not supported yet.

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
Insert with node_handle
insert_return_type insert(node_type&& nh);

If nh is empty, has no effect.

Otherwise inserts the element owned by nh if and only if there is no element in the container with an equivalent key.

Requires:

nh is empty or nh.get_allocator() is equal to the container’s allocator.
Returns:

If nh was empty, returns an insert_return_type with: inserted equal to false, position equal to end() and node empty.

Otherwise if there was already an element with an equivalent key, returns an insert_return_type with: inserted equal to false, position pointing to a matching element and node contains the node from nh.

Otherwise if the insertion succeeded, returns an insert_return_type with: inserted equal to true, position pointing to the newly inserted element and node empty.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

In C++17 this can be used to insert a node extracted from a compatible unordered_multiset, but that is not supported yet.
Insert with Hint and node_handle
iterator insert(const_iterator hint, node_type&& nh);

If nh is empty, has no effect.

Otherwise inserts the element owned by nh if and only if there is no element in the container with an equivalent key.

If there is already an element in the container with an equivalent key has no effect on nh (i.e. nh still contains the node.)

hint is a suggestion to where the element should be inserted.

Requires:

nh is empty or nh.get_allocator() is equal to the container’s allocator.
Returns:

If nh was empty returns end().

If there was already an element in the container with an equivalent key returns an iterator pointing to that.

Otherwise returns an iterator pointing to the newly inserted element.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

This can be used to insert a node extracted from a compatible unordered_multiset.
Erase by Position
iterator erase(iterator position);
iterator erase(const_iterator position);

Erase the element pointed to by position.

Returns:

The iterator following position before the erasure.
Throws:

Only throws an exception if it is thrown by hasher or key_equal.
Notes:

In older versions this could be inefficient because it had to search through several buckets to find the position of the returned iterator. The data structure has been changed so that this is no longer the case, and the alternative erase methods have been deprecated.
Erase by Value
size_type erase(const key_type& k);
template<class K> size_type erase(K&& k);

Erase all elements with key equivalent to k.

Returns:

The number of elements erased.
Throws:

Only throws an exception if it is thrown by hasher or key_equal.
Notes:

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
Erase Range
iterator erase(const_iterator first, const_iterator last);

Erases the elements in the range from first to last.

Returns:

The iterator following the erased elements - i.e. last.
Throws:

Only throws an exception if it is thrown by hasher or key_equal.

In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations.
quick_erase
void quick_erase(const_iterator position);

Erase the element pointed to by position.

Throws:

Only throws an exception if it is thrown by hasher or key_equal.

In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations.
Notes:

This method was implemented because returning an iterator to the next element from erase was expensive, but the container has been redesigned so that is no longer the case. So this method is now deprecated.
erase_return_void
void erase_return_void(const_iterator position);

Erase the element pointed to by position.

Throws:

Only throws an exception if it is thrown by hasher or key_equal.

In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations.
Notes:

This method was implemented because returning an iterator to the next element from erase was expensive, but the container has been redesigned so that is no longer the case. So this method is now deprecated.
swap
void swap(unordered_set& other)
  noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
           boost::is_nothrow_swappable_v<Hash> &&
           boost::is_nothrow_swappable_v<Pred>);

Swaps the contents of the container with the parameter.

If Allocator::propagate_on_container_swap is declared and Allocator::propagate_on_container_swap::value is true then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.

Throws:

Doesn’t throw an exception unless it is thrown by the copy constructor or copy assignment operator of key_equal or hasher.
Notes:

The exception specifications aren’t quite the same as the C++11 standard, as the equality predicate and hash function are swapped using their copy constructors.
clear
void clear() noexcept;

Erases all elements in the container.

Postconditions:

size() == 0
Throws:

Never throws an exception.
merge
template<class H2, class P2>
  void merge(unordered_set<Key, H2, P2, Allocator>& source);
template<class H2, class P2>
  void merge(unordered_set<Key, H2, P2, Allocator>&& source);
template<class H2, class P2>
  void merge(unordered_multiset<Key, H2, P2, Allocator>& source);
template<class H2, class P2>
  void merge(unordered_multiset<Key, H2, P2, Allocator>&& source);

Attempt to "merge" two containers by iterating source and extracting any node in source that is not contained in *this and then inserting it into *this.

Because source can have a different hash function and key equality predicate, the key of each node in source is rehashed using this->hash_function() and then, if required, compared using this->key_eq().

The behavior of this function is undefined if this->get_allocator() != source.get_allocator().

This function does not copy or move any elements and instead simply relocates the nodes from source into *this.

Notes:

  • Pointers and references to transferred elements remain valid.

  • Invalidates iterators to transferred elements.

  • Invalidates iterators belonging to *this.

  • Iterators to non-transferred elements in source remain valid.

Observers

get_allocator
allocator_type get_allocator() const;
hash_function
hasher hash_function() const;
Returns:

The container’s hash function.
key_eq
key_equal key_eq() const;
Returns:

The container’s key equality predicate

Lookup

find
iterator         find(const key_type& k);
const_iterator   find(const key_type& k) const;
template<class K>
  iterator       find(const K& k);
template<class K>
  const_iterator find(const K& k) const;
template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
  iterator       find(CompatibleKey const& k, CompatibleHash const& hash,
                      CompatiblePredicate const& eq);
template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
  const_iterator find(CompatibleKey const& k, CompatibleHash const& hash,
                      CompatiblePredicate const& eq) const;
Returns:

An iterator pointing to an element with key equivalent to k, or b.end() if no such element exists.
Notes:

The templated overloads containing CompatibleKey, CompatibleHash and CompatiblePredicate are non-standard extensions which allow you to use a compatible hash function and equality predicate for a key of a different type in order to avoid an expensive type cast. In general, its use is not encouraged and instead the K member function templates should be used.

The template <typename K> overloads only participate in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
count
size_type        count(const key_type& k) const;
template<class K>
  size_type      count(const K& k) const;
Returns:

The number of elements with key equivalent to k.
Notes:

The template <typename K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
contains
bool             contains(const key_type& k) const;
template<class K>
  bool           contains(const K& k) const;
Returns:

A boolean indicating whether or not there is an element with key equal to key in the container
Notes:

The template <typename K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
equal_range
std::pair<iterator, iterator>               equal_range(const key_type& k);
std::pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
template<class K>
  std::pair<iterator, iterator>             equal_range(const K& k);
template<class K>
  std::pair<const_iterator, const_iterator> equal_range(const K& k) const;
Returns:

A range containing all elements with key equivalent to k. If the container doesn’t contain any such elements, returns std::make_pair(b.end(), b.end()).
Notes:

The template <typename K> overloads only participate in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.

Bucket Interface

bucket_count
size_type bucket_count() const noexcept;
Returns:

The number of buckets.
max_bucket_count
size_type max_bucket_count() const noexcept;
Returns:

An upper bound on the number of buckets.
bucket_size
size_type bucket_size(size_type n) const;
Requires:

n < bucket_count()
Returns:

The number of elements in bucket n.
bucket
size_type bucket(const key_type& k) const;
template<class K> size_type bucket(const K& k) const;
Returns:

The index of the bucket which would contain an element with key k.
Postconditions:

The return value is less than bucket_count().
Notes:

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
begin
local_iterator begin(size_type n);
const_local_iterator begin(size_type n) const;
Requires:

n shall be in the range [0, bucket_count()).
Returns:

A local iterator pointing the first element in the bucket with index n.
end
local_iterator end(size_type n);
const_local_iterator end(size_type n) const;
Requires:

n shall be in the range [0, bucket_count()).
Returns:

A local iterator pointing the 'one past the end' element in the bucket with index n.
cbegin
const_local_iterator cbegin(size_type n) const;
Requires:

n shall be in the range [0, bucket_count()).
Returns:

A constant local iterator pointing the first element in the bucket with index n.
cend
const_local_iterator cend(size_type n) const;
Requires:

n shall be in the range [0, bucket_count()).
Returns:

A constant local iterator pointing the 'one past the end' element in the bucket with index n.

Hash Policy

load_factor
float load_factor() const noexcept;
Returns:

The average number of elements per bucket.
max_load_factor
float max_load_factor() const noexcept;
Returns:

Returns the current maximum load factor.
Set max_load_factor
void max_load_factor(float z);
Effects:

Changes the container’s maximum load factor, using z as a hint.
rehash
void rehash(size_type n);

Changes the number of buckets so that there are at least n buckets, and so that the load factor is less than or equal to the maximum load factor. When applicable, this will either grow or shrink the bucket_count() associated with the container.

When size() == 0, rehash(0) will deallocate the underlying buckets array.

Invalidates iterators, and changes the order of elements. Pointers and references to elements are not invalidated.

Throws:

The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function.
reserve
void reserve(size_type n);

Equivalent to a.rehash(ceil(n / a.max_load_factor())), or a.rehash(1) if n > 0 and a.max_load_factor() == std::numeric_limits<float>::infinity().

Similar to rehash, this function can be used to grow or shrink the number of buckets in the container.

Invalidates iterators, and changes the order of elements. Pointers and references to elements are not invalidated.

Throws:

The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function.

Deduction Guides

A deduction guide will not participate in overload resolution if any of the following are true:

  • It has an InputIterator template parameter and a type that does not qualify as an input iterator is deduced for that parameter.

  • It has an Allocator template parameter and a type that does not qualify as an allocator is deduced for that parameter.

  • It has a Hash template parameter and an integral type or a type that qualifies as an allocator is deduced for that parameter.

  • It has a Pred template parameter and a type that qualifies as an allocator is deduced for that parameter.

A size_­type parameter type in a deduction guide refers to the size_­type member type of the container type deduced by the deduction guide. Its default value coincides with the default value of the constructor selected.

iter-value-type
template<class InputIterator>
  using iter-value-type =
    typename std::iterator_traits<InputIterator>::value_type; // exposition only

Equality Comparisons

operator==
template<class Key, class Hash, class Pred, class Alloc>
  bool operator==(const unordered_set<Key, Hash, Pred, Alloc>& x,
                  const unordered_set<Key, Hash, Pred, Alloc>& y);

Return true if x.size() == y.size() and for every element in x, there is an element in y with the same key, with an equal value (using operator== to compare the value types).

Notes:

The behavior of this function was changed to match the C++11 standard in Boost 1.48.

Behavior is undefined if the two containers don’t have equivalent equality predicates.
operator!=
template<class Key, class Hash, class Pred, class Alloc>
  bool operator!=(const unordered_set<Key, Hash, Pred, Alloc>& x,
                  const unordered_set<Key, Hash, Pred, Alloc>& y);

Return false if x.size() == y.size() and for every element in x, there is an element in y with the same key, with an equal value (using operator== to compare the value types).

Notes:

The behavior of this function was changed to match the C++11 standard in Boost 1.48.

Behavior is undefined if the two containers don’t have equivalent equality predicates.

Swap

template<class Key, class Hash, class Pred, class Alloc>
  void swap(unordered_set<Key, Hash, Pred, Alloc>& x,
            unordered_set<Key, Hash, Pred, Alloc>& y)
    noexcept(noexcept(x.swap(y)));

Swaps the contents of x and y.

If Allocator::propagate_on_container_swap is declared and Allocator::propagate_on_container_swap::value is true then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.

Effects:

x.swap(y)
Throws:

Doesn’t throw an exception unless it is thrown by the copy constructor or copy assignment operator of key_equal or hasher.
Notes:

The exception specifications aren’t quite the same as the C++11 standard, as the equality predicate and hash function are swapped using their copy constructors.

erase_if

template<class K, class H, class P, class A, class Predicate>
  typename unordered_set<K, H, P, A>::size_type
    erase_if(unordered_set<K, H, P, A>& c, Predicate pred);

Traverses the container c and removes all elements for which the supplied predicate returns true.

Returns:

The number of erased elements.
Notes:

Equivalent to:

auto original_size = c.size();
for (auto i = c.begin(), last = c.end(); i != last; ) {
  if (pred(*i)) {
    i = c.erase(i);
  } else {
    ++i;
  }
}
return original_size - c.size();

Class template unordered_multiset

boost::unordered_multiset — An unordered associative container that stores values. The same key can be stored multiple times.

Synopsis

// #include <boost/unordered/unordered_set.hpp>

namespace boost {
  template<class Key,
           class Hash = boost::hash<Key>,
           class Pred = std::equal_to<Key>,
           class Allocator = std::allocator<Key>>
  class unordered_multiset {
  public:
    // types
    using key_type             = Key;
    using value_type           = Key;
    using hasher               = Hash;
    using key_equal            = Pred;
    using allocator_type       = Allocator;
    using pointer              = typename boost::allocator_traits<Allocator>::pointer;
    using const_pointer        = typename boost::allocator_traits<Allocator>::const_pointer;
    using reference            = value_type&;
    using const_reference      = const value_type&;
    using size_type            = implementation-defined;
    using difference_type      = implementation-defined;

    using iterator             = implementation-defined;
    using const_iterator       = implementation-defined;
    using local_iterator       = implementation-defined;
    using const_local_iterator = implementation-defined;
    using node_type            = implementation-defined;

    // construct/copy/destroy
    unordered_multiset();
    explicit unordered_multiset(size_type n,
                                const hasher& hf = hasher(),
                                const key_equal& eql = key_equal(),
                                const allocator_type& a = allocator_type());
    template<class InputIterator>
      unordered_multiset(InputIterator f, InputIterator l,
                         size_type n = implementation-defined,
                         const hasher& hf = hasher(),
                         const key_equal& eql = key_equal(),
                         const allocator_type& a = allocator_type());
    unordered_multiset(const unordered_multiset& other);
    unordered_multiset(unordered_multiset&& other);
    template<class InputIterator>
      unordered_multiset(InputIterator f, InputIterator l, const allocator_type& a);
    explicit unordered_multiset(const Allocator& a);
    unordered_multiset(const unordered_multiset& other, const Allocator& a);
    unordered_multiset(unordered_multiset&& other, const Allocator& a);
    unordered_multiset(std::initializer_list<value_type> il,
                       size_type n = implementation-defined,
                       const hasher& hf = hasher(),
                       const key_equal& eql = key_equal(),
                       const allocator_type& a = allocator_type());
    unordered_multiset(size_type n, const allocator_type& a);
    unordered_multiset(size_type n, const hasher& hf, const allocator_type& a);
    template<class InputIterator>
      unordered_multiset(InputIterator f, InputIterator l, size_type n, const allocator_type& a);
    template<class InputIterator>
      unordered_multiset(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                         const allocator_type& a);
    unordered_multiset(std::initializer_list<value_type> il, const allocator_type& a);
    unordered_multiset(std::initializer_list<value_type> il, size_type n,
                       const allocator_type& a)
    unordered_multiset(std::initializer_list<value_type> il, size_type n, const hasher& hf,
                       const allocator_type& a);
    ~unordered_multiset();
    unordered_multiset& operator=(const unordered_multiset& other);
    unordered_multiset& operator=(unordered_multiset&& other)
      noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
               boost::is_nothrow_move_assignable_v<Hash> &&
               boost::is_nothrow_move_assignable_v<Pred>);
    unordered_multiset& operator=(std::initializer_list<value_type> il);
    allocator_type get_allocator() const noexcept;

    // iterators
    iterator       begin() noexcept;
    const_iterator begin() const noexcept;
    iterator       end() noexcept;
    const_iterator end() const noexcept;
    const_iterator cbegin() const noexcept;
    const_iterator cend() const noexcept;

    // capacity
    [[nodiscard]] bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;

    // modifiers
    template<class... Args> iterator emplace(Args&&... args);
    template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
    iterator insert(const value_type& obj);
    iterator insert(value_type&& obj);
    iterator insert(const_iterator hint, const value_type& obj);
    iterator insert(const_iterator hint, value_type&& obj);
    template<class InputIterator> void insert(InputIterator first, InputIterator last);
    void insert(std::initializer_list<value_type> il);

    node_type extract(const_iterator position);
    node_type extract(const key_type& k);
    template<class K> node_type extract(K&& k);
    iterator insert(node_type&& nh);
    iterator insert(const_iterator hint, node_type&& nh);

    iterator  erase(iterator position);
    iterator  erase(const_iterator position);
    size_type erase(const key_type& k);
    template<class K> size_type erase(K&& x);
    iterator  erase(const_iterator first, const_iterator last);
    void      quick_erase(const_iterator position);
    void      erase_return_void(const_iterator position);
    void      swap(unordered_multiset&)
      noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
               boost::is_nothrow_swappable_v<Hash> &&
               boost::is_nothrow_swappable_v<Pred>);
    void      clear() noexcept;

    template<class H2, class P2>
      void merge(unordered_multiset<Key, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_multiset<Key, H2, P2, Allocator>&& source);
    template<class H2, class P2>
      void merge(unordered_set<Key, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_set<Key, H2, P2, Allocator>&& source);

    // observers
    hasher hash_function() const;
    key_equal key_eq() const;

    // set operations
    iterator         find(const key_type& k);
    const_iterator   find(const key_type& k) const;
    template<class K>
      iterator       find(const K& k);
    template<class K>
      const_iterator find(const K& k) const;
    template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
      iterator       find(CompatibleKey const&, CompatibleHash const&,
                          CompatiblePredicate const&);
    template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
      const_iterator  find(CompatibleKey const&, CompatibleHash const&,
                           CompatiblePredicate const&) const;
    size_type        count(const key_type& k) const;
    template<class K>
      size_type      count(const K& k) const;
    bool             contains(const key_type& k) const;
    template<class K>
      bool           contains(const K& k) const;
    std::pair<iterator, iterator>               equal_range(const key_type& k);
    std::pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
    template<class K>
      std::pair<iterator, iterator>             equal_range(const K& k);
    template<class K>
      std::pair<const_iterator, const_iterator> equal_range(const K& k) const;

    // bucket interface
    size_type bucket_count() const noexcept;
    size_type max_bucket_count() const noexcept;
    size_type bucket_size(size_type n) const;
    size_type bucket(const key_type& k) const;
    template<class K> size_type bucket(const K& k) const;
    local_iterator begin(size_type n);
    const_local_iterator begin(size_type n) const;
    local_iterator end(size_type n);
    const_local_iterator end(size_type n) const;
    const_local_iterator cbegin(size_type n) const;
    const_local_iterator cend(size_type n) const;

    // hash policy
    float load_factor() const noexcept;
    float max_load_factor() const noexcept;
    void max_load_factor(float z);
    void rehash(size_type n);
    void reserve(size_type n);
  };

  // Deduction Guides
  template<class InputIterator,
           class Hash = boost::hash<iter-value-type<InputIterator>>,
           class Pred = std::equal_to<iter-value-type<InputIterator>>,
           class Allocator = std::allocator<iter-value-type<InputIterator>>>
    unordered_multiset(InputIterator, InputIterator, typename see below::size_type = see below,
                       Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_multiset<iter-value-type<InputIterator>, Hash, Pred, Allocator>;

  template<class T, class Hash = boost::hash<T>, class Pred = std::equal_to<T>,
           class Allocator = std::allocator<T>>
    unordered_multiset(std::initializer_list<T>, typename see below::size_type = see below,
                       Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_multiset<T, Hash, Pred, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_multiset(InputIterator, InputIterator, typename see below::size_type, Allocator)
      -> unordered_multiset<iter-value-type<InputIterator>,
                            boost::hash<iter-value-type<InputIterator>>,
                            std::equal_to<iter-value-type<InputIterator>>, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_multiset(InputIterator, InputIterator, Allocator)
      -> unordered_multiset<iter-value-type<InputIterator>,
                            boost::hash<iter-value-type<InputIterator>>,
                            std::equal_to<iter-value-type<InputIterator>>, Allocator>;

  template<class InputIterator, class Hash, class Allocator>
    unordered_multiset(InputIterator, InputIterator, typename see below::size_type, Hash,
                       Allocator)
      -> unordered_multiset<iter-value-type<InputIterator>, Hash,
                            std::equal_to<iter-value-type<InputIterator>>, Allocator>;

  template<class T, class Allocator>
    unordered_multiset(std::initializer_list<T>, typename see below::size_type, Allocator)
      -> unordered_multiset<T, boost::hash<T>, std::equal_to<T>, Allocator>;

  template<class T, class Allocator>
    unordered_multiset(std::initializer_list<T>, Allocator)
      -> unordered_multiset<T, boost::hash<T>, std::equal_to<T>, Allocator>;

  template<class T, class Hash, class Allocator>
    unordered_multiset(std::initializer_list<T>, typename see below::size_type, Hash, Allocator)
      -> unordered_multiset<T, Hash, std::equal_to<T>, Allocator>;

  // Equality Comparisons
  template<class Key, class Hash, class Pred, class Alloc>
    bool operator==(const unordered_multiset<Key, Hash, Pred, Alloc>& x,
                    const unordered_multiset<Key, Hash, Pred, Alloc>& y);

  template<class Key, class Hash, class Pred, class Alloc>
    bool operator!=(const unordered_multiset<Key, Hash, Pred, Alloc>& x,
                    const unordered_multiset<Key, Hash, Pred, Alloc>& y);

  // swap
  template<class Key, class Hash, class Pred, class Alloc>
    void swap(unordered_multiset<Key, Hash, Pred, Alloc>& x,
              unordered_multiset<Key, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));

  template<class K, class H, class P, class A, class Predicate>
    typename unordered_multiset<K, H, P, A>::size_type
      erase_if(unordered_multiset<K, H, P, A>& c, Predicate pred);
}

Description

Template Parameters

Key

Key must be Erasable from the container (i.e. allocator_traits can destroy it).

Hash

A unary function object type that acts a hash function for a Key. It takes a single argument of type Key and returns a value of type std::size_t.

Pred

A binary function object that implements an equivalence relation on values of type Key. A binary function object that induces an equivalence relation on values of type Key. It takes two arguments of type Key and returns a value of type bool.

Allocator

An allocator whose value type is the same as the container’s value type.

The elements are organized into buckets. Keys with the same hash code are stored in the same bucket and elements with equivalent keys are stored next to each other.

The number of buckets can be automatically increased by a call to insert, or as the result of calling rehash.

Typedefs

typedef typename allocator_type::pointer pointer;

value_type* if allocator_type::pointer is not defined.

typedef typename allocator_type::const_pointer const_pointer;

boost::pointer_to_other<pointer, value_type>::type if allocator_type::const_pointer is not defined.

typedef implementation-defined size_type;

An unsigned integral type.

size_type can represent any non-negative value of difference_type.

typedef implementation-defined difference_type;

A signed integral type.

Is identical to the difference type of iterator and const_iterator.

typedef implementation-defined iterator;

A constant iterator whose value type is value_type.

The iterator category is at least a forward iterator.

Convertible to const_iterator.

typedef implementation-defined const_iterator;

A constant iterator whose value type is value_type.

The iterator category is at least a forward iterator.

typedef implementation-defined local_iterator;

An iterator with the same value type, difference type and pointer and reference type as iterator.

A local_iterator object can be used to iterate through a single bucket.

typedef implementation-defined const_local_iterator;

A constant iterator with the same value type, difference type and pointer and reference type as const_iterator.

A const_local_iterator object can be used to iterate through a single bucket.

typedef implementation-defined node_type;

See node_handle_set for details.

Constructors

Default Constructor
unordered_multiset();

Constructs an empty container using hasher() as the hash function, key_equal() as the key equality predicate, allocator_type() as the allocator and a maximum load factor of 1.0.

Postconditions:

size() == 0
Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Bucket Count Constructor
explicit unordered_multiset(size_type n,
                            const hasher& hf = hasher(),
                            const key_equal& eql = key_equal(),
                            const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate, a as the allocator and a maximum load factor of 1.0.

Postconditions:

size() == 0
Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Iterator Range Constructor
template<class InputIterator>
  unordered_multiset(InputIterator f, InputIterator l,
                     size_type n = implementation-defined,
                     const hasher& hf = hasher(),
                     const key_equal& eql = key_equal(),
                     const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate, a as the allocator and a maximum load factor of 1.0 and inserts the elements from [f, l) into it.

Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Copy Constructor
unordered_multiset(const unordered_multiset& other);

The copy constructor. Copies the contained elements, hash function, predicate, maximum load factor and allocator.

If Allocator::select_on_container_copy_construction exists and has the right signature, the allocator will be constructed from its result.

Requires:

value_type is copy constructible
Move Constructor
unordered_multiset(unordered_multiset&& other);

The move constructor.

Notes:

This is implemented using Boost.Move.
Requires:

value_type is move-constructible.

On compilers without rvalue reference support the emulation does not support moving without calling boost::move if value_type is not copyable. So, for example, you can’t return the container from a function.
Iterator Range Constructor with Allocator
template<class InputIterator>
  unordered_multiset(InputIterator f, InputIterator l, const allocator_type& a);

Constructs an empty container using a as the allocator, with the default hash function and key equality predicate and a maximum load factor of 1.0 and inserts the elements from [f, l) into it.

Requires:

hasher, key_equal need to be DefaultConstructible.
Allocator Constructor
explicit unordered_multiset(const Allocator& a);

Constructs an empty container, using allocator a.

Copy Constructor with Allocator
unordered_multiset(const unordered_multiset& other, const Allocator& a);

Constructs an container, copying other's contained elements, hash function, predicate, maximum load factor, but using allocator a.

Move Constructor with Allocator
unordered_multiset(unordered_multiset&& other, const Allocator& a);

Construct a container moving other's contained elements, and having the hash function, predicate and maximum load factor, but using allocate a.

Notes:

This is implemented using Boost.Move.
Requires:

value_type is move insertable.
Initializer List Constructor
unordered_multiset(std::initializer_list<value_type> il,
                   size_type n = implementation-defined,
                   const hasher& hf = hasher(),
                   const key_equal& eql = key_equal(),
                   const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate, a as the allocator and a maximum load factor of 1.0 and inserts the elements from il into it.

Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Bucket Count Constructor with Allocator
unordered_multiset(size_type n, const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, the default hash function and key equality predicate, a as the allocator and a maximum load factor of 1.0.

Postconditions:

size() == 0
Requires:

hasher and key_equal need to be DefaultConstructible.
Bucket Count Constructor with Hasher and Allocator
unordered_multiset(size_type n, const hasher& hf, const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, the default key equality predicate, a as the allocator and a maximum load factor of 1.0.

Postconditions:

size() == 0
Requires:

key_equal needs to be DefaultConstructible.
Iterator Range Constructor with Bucket Count and Allocator
template<class InputIterator>
  unordered_multiset(InputIterator f, InputIterator l, size_type n, const allocator_type& a);

Constructs an empty container with at least n buckets, using a as the allocator, with the default hash function and key equality predicate and a maximum load factor of 1.0 and inserts the elements from [f, l) into it.

Requires:

hasher, key_equal need to be DefaultConstructible.
Iterator Range Constructor with Bucket Count and Hasher
template<class InputIterator>
  unordered_multiset(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                     const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, a as the allocator, with the default key equality predicate and a maximum load factor of 1.0 and inserts the elements from [f, l) into it.

Requires:

key_equal needs to be DefaultConstructible.
initializer_list Constructor with Allocator
unordered_multiset(std::initializer_list<value_type> il, const allocator_type& a);

Constructs an empty container using a as the allocator and a maximum load factor of 1.0 and inserts the elements from il into it.

Requires:

hasher and key_equal need to be DefaultConstructible.
initializer_list Constructor with Bucket Count and Allocator
unordered_multiset(std::initializer_list<value_type> il, size_type n, const allocator_type& a)

Constructs an empty container with at least n buckets, using a as the allocator and a maximum load factor of 1.0 and inserts the elements from il into it.

Requires:

hasher and key_equal need to be DefaultConstructible.
initializer_list Constructor with Bucket Count and Hasher and Allocator
    unordered_multiset(std::initializer_list<value_type> il, size_type n, const hasher& hf,
                       const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, a as the allocator and a maximum load factor of 1.0 and inserts the elements from il into it.

Requires:

key_equal needs to be DefaultConstructible.

Destructor

~unordered_multiset();
Note:

The destructor is applied to every element, and all memory is deallocated

Assignment

Copy Assignment
unordered_multiset& operator=(const unordered_multiset& other);

The assignment operator. Copies the contained elements, hash function, predicate and maximum load factor but not the allocator.

If Alloc::propagate_on_container_copy_assignment exists and Alloc::propagate_on_container_copy_assignment::value is true, the allocator is overwritten, if not the copied elements are created using the existing allocator.

Requires:

value_type is copy constructible
Move Assignment
unordered_multiset& operator=(unordered_multiset&& other)
  noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
           boost::is_nothrow_move_assignable_v<Hash> &&
           boost::is_nothrow_move_assignable_v<Pred>);

The move assignment operator.

If Alloc::propagate_on_container_move_assignment exists and Alloc::propagate_on_container_move_assignment::value is true, the allocator is overwritten, if not the moved elements are created using the existing allocator.

Notes:

On compilers without rvalue references, this is emulated using Boost.Move. Note that on some compilers the copy assignment operator may be used in some circumstances.
Requires:

value_type is move constructible.
Initializer List Assignment
unordered_multiset& operator=(std::initializer_list<value_type> il);

Assign from values in initializer list. All existing elements are either overwritten by the new elements or destroyed.

Requires:

value_type is CopyInsertable into the container and CopyAssignable.

Iterators

begin
iterator       begin() noexcept;
const_iterator begin() const noexcept;
Returns:

An iterator referring to the first element of the container, or if the container is empty the past-the-end value for the container.
end
iterator       end() noexcept;
const_iterator end() const noexcept;
Returns:

An iterator which refers to the past-the-end value for the container.
cbegin
const_iterator cbegin() const noexcept;
Returns:

A const_iterator referring to the first element of the container, or if the container is empty the past-the-end value for the container.
cend
const_iterator cend() const noexcept;
Returns:

A const_iterator which refers to the past-the-end value for the container.

Size and Capacity

empty
[[nodiscard]] bool empty() const noexcept;
Returns:

size() == 0
size
size_type size() const noexcept;
Returns:

std::distance(begin(), end())
max_size
size_type max_size() const noexcept;
Returns:

size() of the largest possible container.

Modifiers

emplace
template<class... Args> iterator emplace(Args&&... args);

Inserts an object, constructed with the arguments args, in the container.

Requires:

value_type is EmplaceConstructible into X from args.
Returns:

An iterator pointing to the inserted element.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

If the compiler doesn’t support variadic template arguments or rvalue references, this is emulated for up to 10 arguments, with no support for rvalue references or move semantics.

Since existing std::pair implementations don’t support std::piecewise_construct this emulates it, but using boost::unordered::piecewise_construct.
emplace_hint
template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);

Inserts an object, constructed with the arguments args, in the container.

hint is a suggestion to where the element should be inserted.

Requires:

value_type is EmplaceConstructible into X from args.
Returns:

An iterator pointing to the inserted element.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

If the compiler doesn’t support variadic template arguments or rvalue references, this is emulated for up to 10 arguments, with no support for rvalue references or move semantics.

Since existing std::pair implementations don’t support std::piecewise_construct this emulates it, but using boost::unordered::piecewise_construct.
Copy Insert
iterator insert(const value_type& obj);

Inserts obj in the container.

Requires:

value_type is CopyInsertable.
Returns:

An iterator pointing to the inserted element.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Move Insert
iterator insert(value_type&& obj);

Inserts obj in the container.

Requires:

value_type is MoveInsertable.
Returns:

An iterator pointing to the inserted element.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Copy Insert with Hint
iterator insert(const_iterator hint, const value_type& obj);

Inserts obj in the container.

hint is a suggestion to where the element should be inserted.

Requires:

value_type is CopyInsertable.
Returns:

An iterator pointing to the inserted element.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Move Insert with Hint
iterator insert(const_iterator hint, value_type&& obj);

Inserts obj in the container.

hint is a suggestion to where the element should be inserted.

Requires:

value_type is MoveInsertable.
Returns:

An iterator pointing to the inserted element.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Insert Iterator Range
template<class InputIterator> void insert(InputIterator first, InputIterator last);

Inserts a range of elements into the container.

Requires:

value_type is EmplaceConstructible into X from *first.
Throws:

When inserting a single element, if an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Insert Initializer List
void insert(std::initializer_list<value_type> il);

Inserts a range of elements into the container. Elements are inserted if and only if there is no element in the container with an equivalent key.

Requires:

value_type is EmplaceConstructible into X from *first.
Throws:

When inserting a single element, if an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.
Extract by Iterator
node_type extract(const_iterator position);

Removes the element pointed to by position.

Returns:

A node_type owning the element.
Notes:

A node extracted using this method can be inserted into a compatible unordered_set.
Extract by Value
node_type extract(const key_type& k);
template<class K> node_type extract(K&& k);

Removes an element with key equivalent to k.

Returns:

A node_type owning the element if found, otherwise an empty node_type.
Throws:

Only throws an exception if it is thrown by hasher or key_equal.
Notes:

A node extracted using this method can be inserted into a compatible unordered_set.

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
Insert with node_handle
iterator insert(node_type&& nh);

If nh is empty, has no effect.

Otherwise inserts the element owned by nh.

Requires:

nh is empty or nh.get_allocator() is equal to the container’s allocator.
Returns:

If nh was empty, returns end().

Otherwise returns an iterator pointing to the newly inserted element.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

This can be used to insert a node extracted from a compatible unordered_set.
Insert with Hint and node_handle
iterator insert(const_iterator hint, node_type&& nh);

If nh is empty, has no effect.

Otherwise inserts the element owned by nh.

hint is a suggestion to where the element should be inserted.

Requires:

nh is empty or nh.get_allocator() is equal to the container’s allocator.
Returns:

If nh was empty, returns end().

Otherwise returns an iterator pointing to the newly inserted element.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

This can be used to insert a node extracted from a compatible unordered_set.
Erase by Position
iterator erase(iterator position);
iterator erase(const_iterator position);

Erase the element pointed to by position.

Returns:

The iterator following position before the erasure.
Throws:

Only throws an exception if it is thrown by hasher or key_equal.
Notes:

In older versions this could be inefficient because it had to search through several buckets to find the position of the returned iterator. The data structure has been changed so that this is no longer the case, and the alternative erase methods have been deprecated.
Erase by Value
size_type erase(const key_type& k);
template<class K> size_type erase(K&& x);

Erase all elements with key equivalent to k.

Returns:

The number of elements erased.
Throws:

Only throws an exception if it is thrown by hasher or key_equal.
Notes:

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
Erase Range
iterator erase(const_iterator first, const_iterator last);

Erases the elements in the range from first to last.

Returns:

The iterator following the erased elements - i.e. last.
Throws:

Only throws an exception if it is thrown by hasher or key_equal.

In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations.
quick_erase
void quick_erase(const_iterator position);

Erase the element pointed to by position.

Throws:

Only throws an exception if it is thrown by hasher or key_equal.

In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations.
Notes:

This method was implemented because returning an iterator to the next element from erase was expensive, but the container has been redesigned so that is no longer the case. So this method is now deprecated.
erase_return_void
void erase_return_void(const_iterator position);

Erase the element pointed to by position.

Throws:

Only throws an exception if it is thrown by hasher or key_equal.

In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations.
Notes:

This method was implemented because returning an iterator to the next element from erase was expensive, but the container has been redesigned so that is no longer the case. So this method is now deprecated.
swap
void swap(unordered_multiset&)
  noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
           boost::is_nothrow_swappable_v<Hash> &&
           boost::is_nothrow_swappable_v<Pred>);

Swaps the contents of the container with the parameter.

If Allocator::propagate_on_container_swap is declared and Allocator::propagate_on_container_swap::value is true then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.

Throws:

Doesn’t throw an exception unless it is thrown by the copy constructor or copy assignment operator of key_equal or hasher.
Notes:

The exception specifications aren’t quite the same as the C++11 standard, as the equality predicate and hash function are swapped using their copy constructors.
clear
void clear() noexcept;

Erases all elements in the container.

Postconditions:

size() == 0
Throws:

Never throws an exception.
merge
template<class H2, class P2>
  void merge(unordered_multiset<Key, H2, P2, Allocator>& source);
template<class H2, class P2>
  void merge(unordered_multiset<Key, H2, P2, Allocator>&& source);
template<class H2, class P2>
  void merge(unordered_set<Key, H2, P2, Allocator>& source);
template<class H2, class P2>
  void merge(unordered_set<Key, H2, P2, Allocator>&& source);

Attempt to "merge" two containers by iterating source and extracting all nodes in source and inserting them into *this.

Because source can have a different hash function and key equality predicate, the key of each node in source is rehashed using this->hash_function() and then, if required, compared using this->key_eq().

The behavior of this function is undefined if this->get_allocator() != source.get_allocator().

This function does not copy or move any elements and instead simply relocates the nodes from source into *this.

Notes:

  • Pointers and references to transferred elements remain valid.

  • Invalidates iterators to transferred elements.

  • Invalidates iterators belonging to *this.

  • Iterators to non-transferred elements in source remain valid.

Observers

get_allocator
allocator_type get_allocator() const noexcept;
hash_function
hasher hash_function() const;
Returns:

The container’s hash function.
key_eq
key_equal key_eq() const;
Returns:

The container’s key equality predicate

Lookup

find
iterator         find(const key_type& k);
const_iterator   find(const key_type& k) const;
template<class K>
  iterator       find(const K& k);
template<class K>
  const_iterator find(const K& k) const;
template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
  iterator       find(CompatibleKey const&, CompatibleHash const&,
                      CompatiblePredicate const&);
template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
  const_iterator  find(CompatibleKey const&, CompatibleHash const&,
                       CompatiblePredicate const&) const;
Returns:

An iterator pointing to an element with key equivalent to k, or b.end() if no such element exists.
Notes:

The templated overloads containing CompatibleKey, CompatibleHash and CompatiblePredicate are non-standard extensions which allow you to use a compatible hash function and equality predicate for a key of a different type in order to avoid an expensive type cast. In general, its use is not encouraged and instead the K member function templates should be used.

The template <typename K> overloads only participate in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
count
size_type        count(const key_type& k) const;
template<class K>
  size_type      count(const K& k) const;
Returns:

The number of elements with key equivalent to k.
Notes:

The template <typename K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
contains
bool             contains(const key_type& k) const;
template<class K>
  bool           contains(const K& k) const;
Returns:

A boolean indicating whether or not there is an element with key equal to key in the container
Notes:

The template <typename K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
equal_range
std::pair<iterator, iterator>               equal_range(const key_type& k);
std::pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
template<class K>
  std::pair<iterator, iterator>             equal_range(const K& k);
template<class K>
  std::pair<const_iterator, const_iterator> equal_range(const K& k) const;
Returns:

A range containing all elements with key equivalent to k. If the container doesn’t contain any such elements, returns std::make_pair(b.end(), b.end()).
Notes:

The template <typename K> overloads only participate in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.

Bucket Interface

bucket_count
size_type bucket_count() const noexcept;
Returns:

The number of buckets.
max_bucket_count
size_type max_bucket_count() const noexcept;
Returns:

An upper bound on the number of buckets.
bucket_size
size_type bucket_size(size_type n) const;
Requires:

n < bucket_count()
Returns:

The number of elements in bucket n.
bucket
size_type bucket(const key_type& k) const;
template<class K> size_type bucket(const K& k) const;
Returns:

The index of the bucket which would contain an element with key k.
Postconditions:

The return value is less than bucket_count().
Notes:

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
begin
local_iterator begin(size_type n);
const_local_iterator begin(size_type n) const;
Requires:

n shall be in the range [0, bucket_count()).
Returns:

A local iterator pointing the first element in the bucket with index n.
end
local_iterator end(size_type n);
const_local_iterator end(size_type n) const;
Requires:

n shall be in the range [0, bucket_count()).
Returns:

A local iterator pointing the 'one past the end' element in the bucket with index n.
cbegin
const_local_iterator cbegin(size_type n) const;
Requires:

n shall be in the range [0, bucket_count()).
Returns:

A constant local iterator pointing the first element in the bucket with index n.
cend
const_local_iterator cend(size_type n) const;
Requires:

n shall be in the range [0, bucket_count()).
Returns:

A constant local iterator pointing the 'one past the end' element in the bucket with index n.

Hash Policy

load_factor
float load_factor() const noexcept;
Returns:

The average number of elements per bucket.
max_load_factor
float max_load_factor() const noexcept;
Returns:

Returns the current maximum load factor.
Set max_load_factor
void max_load_factor(float z);
Effects:

Changes the container’s maximum load factor, using z as a hint.
rehash
void rehash(size_type n);

Changes the number of buckets so that there are at least n buckets, and so that the load factor is less than or equal to the maximum load factor. When applicable, this will either grow or shrink the bucket_count() associated with the container.

When size() == 0, rehash(0) will deallocate the underlying buckets array.

Invalidates iterators, and changes the order of elements. Pointers and references to elements are not invalidated.

Throws:

The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function.
reserve
void reserve(size_type n);

Equivalent to a.rehash(ceil(n / a.max_load_factor())), or a.rehash(1) if n > 0 and a.max_load_factor() == std::numeric_limits<float>::infinity().

Similar to rehash, this function can be used to grow or shrink the number of buckets in the container.

Invalidates iterators, and changes the order of elements. Pointers and references to elements are not invalidated.

Throws:

The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function.

Deduction Guides

A deduction guide will not participate in overload resolution if any of the following are true:

  • It has an InputIterator template parameter and a type that does not qualify as an input iterator is deduced for that parameter.

  • It has an Allocator template parameter and a type that does not qualify as an allocator is deduced for that parameter.

  • It has a Hash template parameter and an integral type or a type that qualifies as an allocator is deduced for that parameter.

  • It has a Pred template parameter and a type that qualifies as an allocator is deduced for that parameter.

A size_­type parameter type in a deduction guide refers to the size_­type member type of the container type deduced by the deduction guide. Its default value coincides with the default value of the constructor selected.

iter-value-type
template<class InputIterator>
  using iter-value-type =
    typename std::iterator_traits<InputIterator>::value_type; // exposition only

Equality Comparisons

operator==
template<class Key, class Hash, class Pred, class Alloc>
  bool operator==(const unordered_multiset<Key, Hash, Pred, Alloc>& x,
                  const unordered_multiset<Key, Hash, Pred, Alloc>& y);

Return true if x.size() == y.size() and for every element in x, there is an element in y with the same key, with an equal value (using operator== to compare the value types).

Notes:

The behavior of this function was changed to match the C++11 standard in Boost 1.48.

Behavior is undefined if the two containers don’t have equivalent equality predicates.
operator!=
template<class Key, class Hash, class Pred, class Alloc>
  bool operator!=(const unordered_multiset<Key, Hash, Pred, Alloc>& x,
                  const unordered_multiset<Key, Hash, Pred, Alloc>& y);

Return false if x.size() == y.size() and for every element in x, there is an element in y with the same key, with an equal value (using operator== to compare the value types).

Notes:

The behavior of this function was changed to match the C++11 standard in Boost 1.48.

Behavior is undefined if the two containers don’t have equivalent equality predicates.

Swap

template<class Key, class Hash, class Pred, class Alloc>
  void swap(unordered_multiset<Key, Hash, Pred, Alloc>& x,
            unordered_multiset<Key, Hash, Pred, Alloc>& y)
    noexcept(noexcept(x.swap(y)));

Swaps the contents of x and y.

If Allocator::propagate_on_container_swap is declared and Allocator::propagate_on_container_swap::value is true then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.

Effects:

x.swap(y)
Throws:

Doesn’t throw an exception unless it is thrown by the copy constructor or copy assignment operator of key_equal or hasher.
Notes:

The exception specifications aren’t quite the same as the C++11 standard, as the equality predicate and hash function are swapped using their copy constructors.

erase_if

template<class K, class H, class P, class A, class Predicate>
  typename unordered_multiset<K, H, P, A>::size_type
    erase_if(unordered_multiset<K, H, P, A>& c, Predicate pred);

Traverses the container c and removes all elements for which the supplied predicate returns true.

Returns:

The number of erased elements.
Notes:

Equivalent to:

auto original_size = c.size();
for (auto i = c.begin(), last = c.end(); i != last; ) {
  if (pred(*i)) {
    i = c.erase(i);
  } else {
    ++i;
  }
}
return original_size - c.size();

Hash traits

Synopsis

// #include <boost/unordered/hash_traits.hpp>

namespace boost {
namespace unordered {

template<typename Hash>
struct hash_is_avalanching;

} // namespace unordered
} // namespace boost

hash_is_avalanching

template<typename Hash>
struct hash_is_avalanching;

A hash function is said to have the avalanching property if small changes in the input translate to large changes in the returned hash code —ideally, flipping one bit in the representation of the input value results in each bit of the hash code flipping with probability 50%. Approaching this property is critical for the proper behavior of open-addressing hash containers.

hash_is_avalanching<Hash>::value is true if Hash::is_avalanching is a valid type, and false otherwise. Users can then declare a hash function Hash as avalanching either by embedding an is_avalanching typedef into the definition of Hash, or directly by specializing hash_is_avalanching<Hash> to a class with an embedded compile-time constant value set to true.

boost::unordered_flat_set and boost::unordered_flat_map use the provided hash function Hash as-is if hash_is_avalanching<Hash>::value is true; otherwise, they implement a bit-mixing post-processing stage to increase the quality of hashing at the expense of extra computational cost.

Class template unordered_flat_map

boost::unordered_flat_map — An open-addressing unordered associative container that associates unique keys with another value.

The performance of boost::unordered_flat_map is much better than that of boost::unordered_map or other implementations of std::unordered_map. Unlike standard unordered associative containers, which are node-based, the elements of a boost::unordered_flat_map are held directly in the bucket array, and insertions into an already occupied bucket are diverted to available buckets in the vicinity of the original position. This type of data layout is known as open addressing.

As a result of its using open addressing, the interface of boost::unordered_flat_map deviates in a number of aspects from that of boost::unordered_flat_map/std::unordered_flat_map:

  • value_type must be move-constructible.

  • Pointer stability is not kept under rehashing.

  • begin() is not constant-time.

  • erase(iterator) returns void.

  • There is no API for bucket handling (except bucket_count) or node extraction/insertion.

  • The maximum load factor of the container is managed internally and can’t be set by the user.

Other than this, boost::unordered_flat_map is mostly a drop-in replacement of node-based standard unordered associative containers.

Synopsis

// #include <boost/unordered/unordered_flat_map.hpp>

namespace boost {
  template<class Key,
           class T,
           class Hash = boost::hash<Key>,
           class Pred = std::equal_to<Key>,
           class Allocator = std::allocator<std::pair<const Key, T>>>
  class unordered_flat_map {
  public:
    // types
    using key_type             = Key;
    using mapped_type          = T;
    using value_type           = std::pair<const Key, T>;
    using init_type            = std::pair<
                                   typename std::remove_const<Key>::type,
                                   typename std::remove_const<T>::type
                                 >;
    using hasher               = Hash;
    using key_equal            = Pred;
    using allocator_type       = Allocator;
    using pointer              = typename std::allocator_traits<Allocator>::pointer;
    using const_pointer        = typename std::allocator_traits<Allocator>::const_pointer;
    using reference            = value_type&;
    using const_reference      = const value_type&;
    using size_type            = std::size_t;
    using difference_type      = std::ptrdiff_t;

    using iterator             = implementation-defined;
    using const_iterator       = implementation-defined;

    // construct/copy/destroy
    unordered_flat_map();
    explicit unordered_flat_map(size_type n,
                                const hasher& hf = hasher(),
                                const key_equal& eql = key_equal(),
                                const allocator_type& a = allocator_type());
    template<class InputIterator>
      unordered_flat_map(InputIterator f, InputIterator l,
                         size_type n = implementation-defined,
                         const hasher& hf = hasher(),
                         const key_equal& eql = key_equal(),
                         const allocator_type& a = allocator_type());
    unordered_flat_map(const unordered_flat_map& other);
    unordered_flat_map(unordered_flat_map&& other);
    template<class InputIterator>
      unordered_flat_map(InputIterator f, InputIterator l, const allocator_type& a);
    explicit unordered_flat_map(const Allocator& a);
    unordered_flat_map(const unordered_flat_map& other, const Allocator& a);
    unordered_flat_map(unordered_flat_map&& other, const Allocator& a);
    unordered_flat_map(std::initializer_list<value_type> il,
                       size_type n = implementation-defined
                       const hasher& hf = hasher(),
                       const key_equal& eql = key_equal(),
                       const allocator_type& a = allocator_type());
    unordered_flat_map(size_type n, const allocator_type& a);
    unordered_flat_map(size_type n, const hasher& hf, const allocator_type& a);
    template<class InputIterator>
      unordered_flat_map(InputIterator f, InputIterator l, size_type n, const allocator_type& a);
    template<class InputIterator>
      unordered_flat_map(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                         const allocator_type& a);
    unordered_flat_map(std::initializer_list<value_type> il, const allocator_type& a);
    unordered_flat_map(std::initializer_list<value_type> il, size_type n,
                       const allocator_type& a);
    unordered_flat_map(std::initializer_list<value_type> il, size_type n, const hasher& hf,
                       const allocator_type& a);
    ~unordered_flat_map();
    unordered_flat_map& operator=(const unordered_flat_map& other);
    unordered_flat_map& operator=(unordered_flat_map&& other)
      noexcept(boost::allocator_traits<Allocator>::is_always_equal::value ||
              boost::allocator_traits<Allocator>::propagate_on_container_move_assignment::value);
    unordered_flat_map& operator=(std::initializer_list<value_type>);
    allocator_type get_allocator() const noexcept;

    // iterators
    iterator       begin() noexcept;
    const_iterator begin() const noexcept;
    iterator       end() noexcept;
    const_iterator end() const noexcept;
    const_iterator cbegin() const noexcept;
    const_iterator cend() const noexcept;

    // capacity
    [[nodiscard]] bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;

    // modifiers
    template<class... Args> std::pair<iterator, bool> emplace(Args&&... args);
    template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
    std::pair<iterator, bool> insert(const value_type& obj);
    std::pair<iterator, bool> insert(const init_type& obj);
    std::pair<iterator, bool> insert(value_type&& obj);
    std::pair<iterator, bool> insert(init_type&& obj);
    iterator       insert(const_iterator hint, const value_type& obj);
    iterator       insert(const_iterator hint, const init_type& obj);
    iterator       insert(const_iterator hint, value_type&& obj);
    iterator       insert(const_iterator hint, init_type&& obj);
    template<class InputIterator> void insert(InputIterator first, InputIterator last);
    void insert(std::initializer_list<value_type>);

    template<class... Args>
      std::pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
    template<class... Args>
      std::pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
    template<class K, class... Args>
      std::pair<iterator, bool> try_emplace(K&& k, Args&&... args);
    template<class... Args>
      iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
    template<class... Args>
      iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);
    template<class K, class... Args>
      iterator try_emplace(const_iterator hint, K&& k, Args&&... args);
    template<class M>
      std::pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
    template<class M>
      std::pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
    template<class K, class M>
      std::pair<iterator, bool> insert_or_assign(K&& k, M&& obj);
    template<class M>
      iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
    template<class M>
      iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);
    template<class K, class M>
      iterator insert_or_assign(const_iterator hint, K&& k, M&& obj);

    void      erase(iterator position);
    void      erase(const_iterator position);
    size_type erase(const key_type& k);
    template<class K> size_type erase(K&& k);
    iterator  erase(const_iterator first, const_iterator last);
    void      swap(unordered_flat_map& other)
      noexcept(boost::allocator_traits<Allocator>::is_always_equal::value ||
               boost::allocator_traits<Allocator>::propagate_on_container_swap::value);
    void      clear() noexcept;

    template<class H2, class P2>
      void merge(unordered_flat_map<Key, T, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_flat_map<Key, T, H2, P2, Allocator>&& source);

    // observers
    hasher hash_function() const;
    key_equal key_eq() const;

    // map operations
    iterator         find(const key_type& k);
    const_iterator   find(const key_type& k) const;
    template<class K>
      iterator       find(const K& k);
    template<class K>
      const_iterator find(const K& k) const;
    size_type        count(const key_type& k) const;
    template<class K>
      size_type      count(const K& k) const;
    bool             contains(const key_type& k) const;
    template<class K>
      bool           contains(const K& k) const;
    std::pair<iterator, iterator>               equal_range(const key_type& k);
    std::pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
    template<class K>
      std::pair<iterator, iterator>             equal_range(const K& k);
    template<class K>
      std::pair<const_iterator, const_iterator> equal_range(const K& k) const;

    // element access
    mapped_type& operator[](const key_type& k);
    mapped_type& operator[](key_type&& k);
    template<class K> mapped_type& operator[](K&& k);
    mapped_type& at(const key_type& k);
    const mapped_type& at(const key_type& k) const;
    template<class K> mapped_type& at(const K& k);
    template<class K> const mapped_type& at(const K& k) const;

    // bucket interface
    size_type bucket_count() const noexcept;

    // hash policy
    float load_factor() const noexcept;
    float max_load_factor() const noexcept;
    void max_load_factor(float z);
    size_type max_load() const noexcept;
    void rehash(size_type n);
    void reserve(size_type n);
  };

  // Deduction Guides
  template<class InputIterator,
           class Hash = boost::hash<iter-key-type<InputIterator>>,
           class Pred = std::equal_to<iter-key-type<InputIterator>>,
           class Allocator = std::allocator<iter-to-alloc-type<InputIterator>>>
    unordered_flat_map(InputIterator, InputIterator, typename see below::size_type = see below,
                       Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_flat_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Hash,
                            Pred, Allocator>;

  template<class Key, class T, class Hash = boost::hash<Key>,
           class Pred = std::equal_to<Key>,
           class Allocator = std::allocator<std::pair<const Key, T>>>
    unordered_flat_map(std::initializer_list<std::pair<Key, T>>,
                       typename see below::size_type = see below, Hash = Hash(),
                       Pred = Pred(), Allocator = Allocator())
      -> unordered_flat_map<Key, T, Hash, Pred, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_flat_map(InputIterator, InputIterator, typename see below::size_type, Allocator)
      -> unordered_flat_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
                            boost::hash<iter-key-type<InputIterator>>,
                            std::equal_to<iter-key-type<InputIterator>>, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_flat_map(InputIterator, InputIterator, Allocator)
      -> unordered_flat_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
                            boost::hash<iter-key-type<InputIterator>>,
                            std::equal_to<iter-key-type<InputIterator>>, Allocator>;

  template<class InputIterator, class Hash, class Allocator>
    unordered_flat_map(InputIterator, InputIterator, typename see below::size_type, Hash,
                       Allocator)
      -> unordered_flat_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Hash,
                            std::equal_to<iter-key-type<InputIterator>>, Allocator>;

  template<class Key, class T, class Allocator>
    unordered_flat_map(std::initializer_list<std::pair<Key, T>>, typename see below::size_type,
                       Allocator)
      -> unordered_flat_map<Key, T, boost::hash<Key>, std::equal_to<Key>, Allocator>;

  template<class Key, class T, class Allocator>
    unordered_flat_map(std::initializer_list<std::pair<Key, T>>, Allocator)
      -> unordered_flat_map<Key, T, boost::hash<Key>, std::equal_to<Key>, Allocator>;

  template<class Key, class T, class Hash, class Allocator>
    unordered_flat_map(std::initializer_list<std::pair<Key, T>>, typename see below::size_type,
                       Hash, Allocator)
      -> unordered_flat_map<Key, T, Hash, std::equal_to<Key>, Allocator>;

  // Equality Comparisons
  template<class Key, class T, class Hash, class Pred, class Alloc>
    bool operator==(const unordered_flat_map<Key, T, Hash, Pred, Alloc>& x,
                    const unordered_flat_map<Key, T, Hash, Pred, Alloc>& y);

  template<class Key, class T, class Hash, class Pred, class Alloc>
    bool operator!=(const unordered_flat_map<Key, T, Hash, Pred, Alloc>& x,
                    const unordered_flat_map<Key, T, Hash, Pred, Alloc>& y);

  // swap
  template<class Key, class T, class Hash, class Pred, class Alloc>
    void swap(unordered_flat_map<Key, T, Hash, Pred, Alloc>& x,
              unordered_flat_map<Key, T, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));

  template<class K, class T, class H, class P, class A, class Predicate>
    typename unordered_flat_map<K, T, H, P, A>::size_type
       erase_if(unordered_flat_map<K, T, H, P, A>& c, Predicate pred);
}

Description

Template Parameters

Key

Key and T must be MoveConstructible. std::pair<const Key, T> must be EmplaceConstructible into the container from any std::pair object convertible to it, and it also must be Erasable from the container.

T

Hash

A unary function object type that acts a hash function for a Key. It takes a single argument of type Key and returns a value of type std::size_t.

Pred

A binary function object that induces an equivalence relation on values of type Key. It takes two arguments of type Key and returns a value of type bool.

Allocator

An allocator whose value type is the same as the container’s value type. std::allocator_traits<Allocator>::pointer and std::allocator_traits<Allocator>::const_pointer must be convertible to/from value_type* and const value_type*, respectively.

The elements of the container are held into an internal bucket array. An element is inserted into a bucket determined by its hash code, but if the bucket is already occupied (a collision), an available one in the vicinity of the original position is used.

The size of the bucket array can be automatically increased by a call to insert/emplace, or as a result of calling rehash/reserve. The load factor of the container (number of elements divided by number of buckets) is never greater than max_load_factor(), except possibly for small sizes where the implementation may decide to allow for higher loads.

If hash_is_avalanching<Hash>::value is true, the hash function is used as-is; otherwise, a bit-mixing post-processing stage is added to increase the quality of hashing at the expense of extra computational cost.

Typedefs

typedef implementation-defined iterator;

An iterator whose value type is value_type.

The iterator category is at least a forward iterator.

Convertible to const_iterator.

typedef implementation-defined const_iterator;

A constant iterator whose value type is value_type.

The iterator category is at least a forward iterator.

Constructors

Default Constructor
unordered_flat_map();

Constructs an empty container using hasher() as the hash function, key_equal() as the key equality predicate and allocator_type() as the allocator.

Postconditions:

size() == 0
Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Bucket Count Constructor
explicit unordered_flat_map(size_type n,
                            const hasher& hf = hasher(),
                            const key_equal& eql = key_equal(),
                            const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate, and a as the allocator.

Postconditions:

size() == 0
Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Iterator Range Constructor
template<class InputIterator>
  unordered_flat_map(InputIterator f, InputIterator l,
                     size_type n = implementation-defined,
                     const hasher& hf = hasher(),
                     const key_equal& eql = key_equal(),
                     const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate and a as the allocator, and inserts the elements from [f, l) into it.

Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Copy Constructor
unordered_flat_map(unordered_flat_map const& other);

The copy constructor. Copies the contained elements, hash function, predicate and allocator.

If Allocator::select_on_container_copy_construction exists and has the right signature, the allocator will be constructed from its result.

Requires:

value_type is copy constructible
Move Constructor
unordered_flat_map(unordered_flat_map&& other);

The move constructor. The internal bucket array of other is transferred directly to the new container. The hash function, predicate and allocator are moved-constructed from other.

Iterator Range Constructor with Allocator
template<class InputIterator>
  unordered_flat_map(InputIterator f, InputIterator l, const allocator_type& a);

Constructs an empty container using a as the allocator, with the default hash function and key equality predicate and inserts the elements from [f, l) into it.

Requires:

hasher, key_equal need to be DefaultConstructible.
Allocator Constructor
explicit unordered_flat_map(Allocator const& a);

Constructs an empty container, using allocator a.

Copy Constructor with Allocator
unordered_flat_map(unordered_flat_map const& other, Allocator const& a);

Constructs a container, copying other's contained elements, hash function, and predicate, but using allocator a.

Move Constructor with Allocator
unordered_flat_map(unordered_flat_map&& other, Allocator const& a);

If a == other.get_allocator(), the elements of other are transferred directly to the new container; otherwise, elements are moved-constructed from those of other. The hash function and predicate are moved-constructed from other, and the allocator is copy-constructed from a.

Initializer List Constructor
unordered_flat_map(std::initializer_list<value_type> il,
              size_type n = implementation-defined
              const hasher& hf = hasher(),
              const key_equal& eql = key_equal(),
              const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate and a, and inserts the elements from il into it.

Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Bucket Count Constructor with Allocator
unordered_flat_map(size_type n, allocator_type const& a);

Constructs an empty container with at least n buckets, using hf as the hash function, the default hash function and key equality predicate and a as the allocator.

Postconditions:

size() == 0
Requires:

hasher and key_equal need to be DefaultConstructible.
Bucket Count Constructor with Hasher and Allocator
unordered_flat_map(size_type n, hasher const& hf, allocator_type const& a);

Constructs an empty container with at least n buckets, using hf as the hash function, the default key equality predicate and a as the allocator.

Postconditions:

size() == 0
Requires:

key_equal needs to be DefaultConstructible.
Iterator Range Constructor with Bucket Count and Allocator
template<class InputIterator>
  unordered_flat_map(InputIterator f, InputIterator l, size_type n, const allocator_type& a);

Constructs an empty container with at least n buckets, using a as the allocator and default hash function and key equality predicate, and inserts the elements from [f, l) into it.

Requires:

hasher, key_equal need to be DefaultConstructible.
Iterator Range Constructor with Bucket Count and Hasher
    template<class InputIterator>
      unordered_flat_map(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                         const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, a as the allocator, with the default key equality predicate, and inserts the elements from [f, l) into it.

Requires:

key_equal needs to be DefaultConstructible.
initializer_list Constructor with Allocator
unordered_flat_map(std::initializer_list<value_type> il, const allocator_type& a);

Constructs an empty container using a and default hash function and key equality predicate, and inserts the elements from il into it.

Requires:

hasher and key_equal need to be DefaultConstructible.
initializer_list Constructor with Bucket Count and Allocator
unordered_flat_map(std::initializer_list<value_type> il, size_type n, const allocator_type& a);

Constructs an empty container with at least n buckets, using a and default hash function and key equality predicate, and inserts the elements from il into it.

Requires:

hasher and key_equal need to be DefaultConstructible.
initializer_list Constructor with Bucket Count and Hasher and Allocator
unordered_flat_map(std::initializer_list<value_type> il, size_type n, const hasher& hf,
                   const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, a as the allocator and default key equality predicate,and inserts the elements from il into it.

Requires:

key_equal needs to be DefaultConstructible.

Destructor

~unordered_flat_map();
Note:

The destructor is applied to every element, and all memory is deallocated

Assignment

Copy Assignment
unordered_flat_map& operator=(unordered_flat_map const& other);

The assignment operator. Destroys previously existing elements, copy-assigns the hash function and predicate from other, copy-assigns the allocator from other if Alloc::propagate_on_container_copy_assignment exists and Alloc::propagate_on_container_copy_assignment::value is true, and finally inserts copies of the elements of other.

Requires:

value_type is CopyInsertable
Move Assignment
unordered_flat_map& operator=(unordered_flat_map&& other)
  noexcept(boost::allocator_traits<Allocator>::is_always_equal::value ||
           boost::allocator_traits<Allocator>::propagate_on_container_move_assignment::value);

The move assignment operator. Destroys previously existing elements, swaps the hash function and predicate from other, and move-assigns the allocator from other if Alloc::propagate_on_container_move_assignment exists and Alloc::propagate_on_container_move_assignment::value is true. If at this point the allocator is equal to other.get_allocator(), the internal bucket array of other is transferred directly to the new container; otherwise, inserts move-constructed copies of the elements of other.

Initializer List Assignment
unordered_flat_map& operator=(std::initializer_list<value_type> il);

Assign from values in initializer list. All previously existing elements are destroyed.

Requires:

value_type is CopyInsertable

Iterators

begin
iterator begin() noexcept;
const_iterator begin() const noexcept;
Returns:

An iterator referring to the first element of the container, or if the container is empty the past-the-end value for the container.
Complexity:

O(bucket_count())
end
iterator end() noexcept;
const_iterator end() const noexcept;
Returns:

An iterator which refers to the past-the-end value for the container.
cbegin
const_iterator cbegin() const noexcept;
Returns:

A const_iterator referring to the first element of the container, or if the container is empty the past-the-end value for the container.
Complexity:

O(bucket_count())
cend
const_iterator cend() const noexcept;
Returns:

A const_iterator which refers to the past-the-end value for the container.

Size and Capacity

empty
[[nodiscard]] bool empty() const noexcept;
Returns:

size() == 0
size
size_type size() const noexcept;
Returns:

std::distance(begin(), end())
max_size
size_type max_size() const noexcept;
Returns:

size() of the largest possible container.

Modifiers

emplace
template<class... Args> std::pair<iterator, bool> emplace(Args&&... args);

Inserts an object, constructed with the arguments args, in the container if and only if there is no element in the container with an equivalent key.

Requires:

value_type is constructible from args.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.
emplace_hint
    template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);

Inserts an object, constructed with the arguments args, in the container if and only if there is no element in the container with an equivalent key.

position is a suggestion to where the element should be inserted. This implementation ignores it.

Requires:

value_type is constructible from args.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.
Copy Insert
std::pair<iterator, bool> insert(const value_type& obj);
std::pair<iterator, bool> insert(const init_type& obj);

Inserts obj in the container if and only if there is no element in the container with an equivalent key.

Requires:

value_type is CopyInsertable.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.

A call of the form insert(x), where x is equally convertible to both const value_type& and const init_type&, is not ambiguous and selects the init_type overload.
Move Insert
std::pair<iterator, bool> insert(value_type&& obj);
std::pair<iterator, bool> insert(init_type&& obj);

Inserts obj in the container if and only if there is no element in the container with an equivalent key.

Requires:

value_type is MoveInsertable.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.

A call of the form insert(x), where x is equally convertible to both value_type&& and init_type&&, is not ambiguous and selects the init_type overload.
Copy Insert with Hint
iterator insert(const_iterator hint, const value_type& obj);
iterator insert(const_iterator hint, const init_type& obj);

Inserts obj in the container if and only if there is no element in the container with an equivalent key.

hint is a suggestion to where the element should be inserted. This implementation ignores it.

Requires:

value_type is CopyInsertable.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.

A call of the form insert(hint, x), where x is equally convertible to both const value_type& and const init_type&, is not ambiguous and selects the init_type overload.
Move Insert with Hint
iterator insert(const_iterator hint, value_type&& obj);
iterator insert(const_iterator hint, init_type&& obj);

Inserts obj in the container if and only if there is no element in the container with an equivalent key.

hint is a suggestion to where the element should be inserted. This implementation ignores it.

Requires:

value_type is MoveInsertable.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.

A call of the form insert(hint, x), where x is equally convertible to both value_type&& and init_type&&, is not ambiguous and selects the init_type overload.
Insert Iterator Range
template<class InputIterator> void insert(InputIterator first, InputIterator last);

Inserts a range of elements into the container. Elements are inserted if and only if there is no element in the container with an equivalent key.

Requires:

value_type is EmplaceConstructible into the container from *first.
Throws:

When inserting a single element, if an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.
Insert Initializer List
void insert(std::initializer_list<value_type>);

Inserts a range of elements into the container. Elements are inserted if and only if there is no element in the container with an equivalent key.

Requires:

value_type is EmplaceConstructible into the container from *first.
Throws:

When inserting a single element, if an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.
try_emplace
template<class... Args>
  std::pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
template<class... Args>
  std::pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
template<class K, class... Args>
  std::pair<iterator, bool> try_emplace(K&& k, Args&&... args);

Inserts a new node into the container if there is no existing element with key k contained within it.

If there is an existing element with key k this function does nothing.

Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

This function is similiar to emplace, with the difference that no value_type is constructed if there is an element with an equivalent key; otherwise, the construction is of the form:

// first two overloads
value_type(std::piecewise_construct,
           std::forward_as_tuple(boost::forward<Key>(k)),
           std::forward_as_tuple(boost::forward<Args>(args)...))

// third overload
value_type(std::piecewise_construct,
           std::forward_as_tuple(boost::forward<K>(k)),
           std::forward_as_tuple(boost::forward<Args>(args)...))

unlike emplace, which simply forwards all arguments to value_type's constructor.

Can invalidate iterators pointers and references, but only if the insert causes the load to be greater than the maximum load.

The template <class K, class…​ Args> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
try_emplace with Hint
template<class... Args>
  iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
template<class... Args>
  iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);
template<class K, class... Args>
  iterator try_emplace(const_iterator hint, K&& k, Args&&... args);

Inserts a new node into the container if there is no existing element with key k contained within it.

If there is an existing element with key k this function does nothing.

hint is a suggestion to where the element should be inserted. This implementation ignores it.

Returns:

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

This function is similiar to emplace_hint, with the difference that no value_type is constructed if there is an element with an equivalent key; otherwise, the construction is of the form:

// first two overloads
value_type(std::piecewise_construct,
           std::forward_as_tuple(boost::forward<Key>(k)),
           std::forward_as_tuple(boost::forward<Args>(args)...))

// third overload
value_type(std::piecewise_construct,
           std::forward_as_tuple(boost::forward<K>(k)),
           std::forward_as_tuple(boost::forward<Args>(args)...))

unlike emplace_hint, which simply forwards all arguments to value_type's constructor.

Can invalidate iterators pointers and references, but only if the insert causes the load to be greater than the maximum load.

The template <class K, class…​ Args> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
insert_or_assign
template<class M>
  std::pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
template<class M>
  std::pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
template<class K, class M>
  std::pair<iterator, bool> insert_or_assign(K&& k, M&& obj);

Inserts a new element into the container or updates an existing one by assigning to the contained value.

If there is an element with key k, then it is updated by assigning boost::forward<M>(obj).

If there is no such element, it is added to the container as:

// first two overloads
value_type(std::piecewise_construct,
           std::forward_as_tuple(boost::forward<Key>(k)),
           std::forward_as_tuple(boost::forward<M>(obj)))

// third overload
value_type(std::piecewise_construct,
           std::forward_as_tuple(boost::forward<K>(k)),
           std::forward_as_tuple(boost::forward<M>(obj)))
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators pointers and references, but only if the insert causes the load to be greater than the maximum load.

The template<class K, class M> only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
insert_or_assign with Hint
template<class M>
  iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
template<class M>
  iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);
template<class K, class M>
  iterator insert_or_assign(const_iterator hint, K&& k, M&& obj);

Inserts a new element into the container or updates an existing one by assigning to the contained value.

If there is an element with key k, then it is updated by assigning boost::forward<M>(obj).

If there is no such element, it is added to the container as:

// first two overloads
value_type(std::piecewise_construct,
           std::forward_as_tuple(boost::forward<Key>(k)),
           std::forward_as_tuple(boost::forward<M>(obj)))

// third overload
value_type(std::piecewise_construct,
           std::forward_as_tuple(boost::forward<K>(k)),
           std::forward_as_tuple(boost::forward<M>(obj)))

hint is a suggestion to where the element should be inserted. This implementation ignores it.

Returns:

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.

The template<class K, class M> only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
Erase by Position
void erase(iterator position);
void erase(const_iterator position);

Erase the element pointed to by position.

Throws:

Nothing.
Erase by Key
size_type erase(const key_type& k);
template<class K> size_type erase(K&& k);

Erase all elements with key equivalent to k.

Returns:

The number of elements erased.
Throws:

Only throws an exception if it is thrown by hasher or key_equal.
Notes:

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
Erase Range
iterator erase(const_iterator first, const_iterator last);

Erases the elements in the range from first to last.

Returns:

The iterator following the erased elements - i.e. last.
Throws:

Nothing in this implementation (neither the hasher nor the key_equal objects are called).
swap
void swap(unordered_flat_map& other)
  noexcept(boost::allocator_traits<Allocator>::is_always_equal::value ||
           boost::allocator_traits<Allocator>::propagate_on_container_swap::value);

Swaps the contents of the container with the parameter.

If Allocator::propagate_on_container_swap is declared and Allocator::propagate_on_container_swap::value is true then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.

Throws:

Nothing unless key_equal or hasher throw on swapping.
clear
void clear() noexcept;

Erases all elements in the container.

Postconditions:

size() == 0, max_load() >= max_load_factor() * bucket_count()
merge
template<class H2, class P2>
  void merge(unordered_flat_map<Key, T, H2, P2, Allocator>& source);
template<class H2, class P2>
  void merge(unordered_flat_map<Key, T, H2, P2, Allocator>&& source);

Move-inserts all the elements from source whose key is not already present in *this, and erases them from source.

Observers

get_allocator
allocator_type get_allocator() const noexcept;
Returns:

The container’s allocator.
hash_function
hasher hash_function() const;
Returns:

The container’s hash function.
key_eq
key_equal key_eq() const;
Returns:

The container’s key equality predicate

Lookup

find
iterator         find(const key_type& k);
const_iterator   find(const key_type& k) const;
template<class K>
  iterator       find(const K& k);
Returns:

An iterator pointing to an element with key equivalent to k, or end() if no such element exists.
Notes:

The template <typename K> overloads only participate in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
count
size_type        count(const key_type& k) const;
template<class K>
  size_type      count(const K& k) const;
Returns:

The number of elements with key equivalent to k.
Notes:

The template <typename K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
contains
bool             contains(const key_type& k) const;
template<class K>
  bool           contains(const K& k) const;
Returns:

A boolean indicating whether or not there is an element with key equal to key in the container
Notes:

The template <typename K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
equal_range
std::pair<iterator, iterator>               equal_range(const key_type& k);
std::pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
template<class K>
  std::pair<iterator, iterator>             equal_range(const K& k);
template<class K>
  std::pair<const_iterator, const_iterator> equal_range(const K& k) const;
Returns:

A range containing all elements with key equivalent to k. If the container doesn’t contain any such elements, returns std::make_pair(b.end(), b.end()).
Notes:

The template <typename K> overloads only participate in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
operator[]
mapped_type& operator[](const key_type& k);
mapped_type& operator[](key_type&& k);
template<class K> mapped_type& operator[](K&& k);
Effects:

If the container does not already contain an element with a key equivalent to k, inserts the value std::pair<key_type const, mapped_type>(k, mapped_type()).
Returns:

A reference to x.second where x is the element already in the container, or the newly inserted element with a key equivalent to k.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
at
mapped_type& at(const key_type& k);
const mapped_type& at(const key_type& k) const;
template<class K> mapped_type& at(const K& k);
template<class K> const mapped_type& at(const K& k) const;
Returns:

A reference to x.second where x is the (unique) element whose key is equivalent to k.
Throws:

An exception object of type std::out_of_range if no such element is present.
Notes:

The template<class K> overloads only participate in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.

Bucket Interface

bucket_count
size_type bucket_count() const noexcept;
Returns:

The size of the bucket array.

Hash Policy

load_factor
float load_factor() const noexcept;
Returns:

static_cast<float>(size())/static_cast<float>(bucket_count()), or 0 if bucket_count() == 0.
max_load_factor
float max_load_factor() const noexcept;
Returns:

Returns the container’s maximum load factor.
Set max_load_factor
void max_load_factor(float z);
Effects:

Does nothing, as the user is not allowed to change this parameter. Kept for compatibility with boost::unordered_map.
max_load
size_type max_load() const noexcept;
Returns:

The maximum number of elements the container can hold without rehashing, assuming that no further elements will be erased.
Note:

After construction, rehash or clearance, the container’s maximum load is at least max_load_factor() * bucket_count(). This number may decrease on erasure under high-load conditions.
rehash
void rehash(size_type n);

Changes if necessary the size of the bucket array so that there are at least n buckets, and so that the load factor is less than or equal to the maximum load factor. When applicable, this will either grow or shrink the bucket_count() associated with the container.

When size() == 0, rehash(0) will deallocate the underlying buckets array.

Invalidates iterators, pointers and references, and changes the order of elements.

Throws:

The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function.
reserve
void reserve(size_type n);

Equivalent to a.rehash(ceil(n / a.max_load_factor())).

Similar to rehash, this function can be used to grow or shrink the number of buckets in the container.

Invalidates iterators, pointers and references, and changes the order of elements.

Throws:

The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function.

Deduction Guides

A deduction guide will not participate in overload resolution if any of the following are true:

  • It has an InputIterator template parameter and a type that does not qualify as an input iterator is deduced for that parameter.

  • It has an Allocator template parameter and a type that does not qualify as an allocator is deduced for that parameter.

  • It has a Hash template parameter and an integral type or a type that qualifies as an allocator is deduced for that parameter.

  • It has a Pred template parameter and a type that qualifies as an allocator is deduced for that parameter.

A size_­type parameter type in a deduction guide refers to the size_­type member type of the container type deduced by the deduction guide. Its default value coincides with the default value of the constructor selected.

iter-value-type
template<class InputIterator>
  using iter-value-type =
    typename std::iterator_traits<InputIterator>::value_type; // exposition only
iter-key-type
template<class InputIterator>
  using iter-key-type = std::remove_const_t<
    std::tuple_element_t<0, iter-value-type<InputIterator>>>; // exposition only
iter-mapped-type
template<class InputIterator>
  using iter-mapped-type =
    std::tuple_element_t<1, iter-value-type<InputIterator>>;  // exposition only
iter-to-alloc-type
template<class InputIterator>
  using iter-to-alloc-type = std::pair<
    std::add_const_t<std::tuple_element_t<0, iter-value-type<InputIterator>>>,
    std::tuple_element_t<1, iter-value-type<InputIterator>>>; // exposition only

Equality Comparisons

operator==
template<class Key, class T, class Hash, class Pred, class Alloc>
  bool operator==(const unordered_flat_map<Key, T, Hash, Pred, Alloc>& x,
                  const unordered_flat_map<Key, T, Hash, Pred, Alloc>& y);

Return true if x.size() == y.size() and for every element in x, there is an element in y with the same key, with an equal value (using operator== to compare the value types).

Notes:

Behavior is undefined if the two containers don’t have equivalent equality predicates.
operator!=
template<class Key, class T, class Hash, class Pred, class Alloc>
  bool operator!=(const unordered_flat_map<Key, T, Hash, Pred, Alloc>& x,
                  const unordered_flat_map<Key, T, Hash, Pred, Alloc>& y);

Return false if x.size() == y.size() and for every element in x, there is an element in y with the same key, with an equal value (using operator== to compare the value types).

Notes:

Behavior is undefined if the two containers don’t have equivalent equality predicates.

Swap

template<class Key, class T, class Hash, class Pred, class Alloc>
  void swap(unordered_flat_map<Key, T, Hash, Pred, Alloc>& x,
            unordered_flat_map<Key, T, Hash, Pred, Alloc>& y)
    noexcept(noexcept(x.swap(y)));

Swaps the contents of x and y.

If Allocator::propagate_on_container_swap is declared and Allocator::propagate_on_container_swap::value is true then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.

Effects:

x.swap(y)
Throws:

Nothing unless key_equal or hasher throw on swapping.

erase_if

template<class K, class T, class H, class P, class A, class Predicate>
  typename unordered_flat_map<K, T, H, P, A>::size_type
    erase_if(unordered_flat_map<K, T, H, P, A>& c, Predicate pred);

Traverses the container c and removes all elements for which the supplied predicate returns true.

Returns:

The number of erased elements.
Notes:

Equivalent to:

auto original_size = c.size();
for (auto i = c.begin(), last = c.end(); i != last; ) {
  if (pred(*i)) {
    i = c.erase(i);
  } else {
    ++i;
  }
}
return original_size - c.size();

Class template unordered_flat_set

boost::unordered_flat_set — An open-addressing unordered associative container that stores unique values.

The performance of boost::unordered_flat_set is much better than that of boost::unordered_set or other implementations of std::unordered_set. Unlike standard unordered associative containers, which are node-based, the elements of a boost::unordered_flat_set are held directly in the bucket array, and insertions into an already occupied bucket are diverted to available buckets in the vicinity of the original position. This type of data layout is known as open addressing.

As a result of its using open addressing, the interface of boost::unordered_flat_set deviates in a number of aspects from that of boost::unordered_flat_set/std::unordered_flat_set:

  • value_type must be move-constructible.

  • Pointer stability is not kept under rehashing.

  • begin() is not constant-time.

  • erase(iterator) returns void.

  • There is no API for bucket handling (except bucket_count) or node extraction/insertion.

  • The maximum load factor of the container is managed internally and can’t be set by the user.

Other than this, boost::unordered_flat_set is mostly a drop-in replacement of node-based standard unordered associative containers.

Synopsis

// #include <boost/unordered/unordered_flat_set.hpp>

namespace boost {
  template<class Key,
           class Hash = boost::hash<Key>,
           class Pred = std::equal_to<Key>,
           class Allocator = std::allocator<Key>>
  class unordered_flat_set {
  public:
    // types
    using key_type             = Key;
    using value_type           = Key;
    using init_type            = Key;
    using hasher               = Hash;
    using key_equal            = Pred;
    using allocator_type       = Allocator;
    using pointer              = typename std::allocator_traits<Allocator>::pointer;
    using const_pointer        = typename std::allocator_traits<Allocator>::const_pointer;
    using reference            = value_type&;
    using const_reference      = const value_type&;
    using size_type            = std::size_t;
    using difference_type      = std::ptrdiff_t;

    using iterator             = implementation-defined;
    using const_iterator       = implementation-defined;

    // construct/copy/destroy
    unordered_flat_set();
    explicit unordered_flat_set(size_type n,
                                const hasher& hf = hasher(),
                                const key_equal& eql = key_equal(),
                                const allocator_type& a = allocator_type());
    template<class InputIterator>
      unordered_flat_set(InputIterator f, InputIterator l,
                         size_type n = implementation-defined,
                         const hasher& hf = hasher(),
                         const key_equal& eql = key_equal(),
                         const allocator_type& a = allocator_type());
    unordered_flat_set(const unordered_flat_set& other);
    unordered_flat_set(unordered_flat_set&& other);
    template<class InputIterator>
      unordered_flat_set(InputIterator f, InputIterator l, const allocator_type& a);
    explicit unordered_flat_set(const Allocator& a);
    unordered_flat_set(const unordered_flat_set& other, const Allocator& a);
    unordered_flat_set(unordered_flat_set&& other, const Allocator& a);
    unordered_flat_set(std::initializer_list<value_type> il,
                       size_type n = implementation-defined
                       const hasher& hf = hasher(),
                       const key_equal& eql = key_equal(),
                       const allocator_type& a = allocator_type());
    unordered_flat_set(size_type n, const allocator_type& a);
    unordered_flat_set(size_type n, const hasher& hf, const allocator_type& a);
    template<class InputIterator>
      unordered_flat_set(InputIterator f, InputIterator l, size_type n, const allocator_type& a);
    template<class InputIterator>
      unordered_flat_set(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                         const allocator_type& a);
    unordered_flat_set(std::initializer_list<value_type> il, const allocator_type& a);
    unordered_flat_set(std::initializer_list<value_type> il, size_type n,
                       const allocator_type& a);
    unordered_flat_set(std::initializer_list<value_type> il, size_type n, const hasher& hf,
                       const allocator_type& a);
    ~unordered_flat_set();
    unordered_flat_set& operator=(const unordered_flat_set& other);
    unordered_flat_set& operator=(unordered_flat_set&& other)
      noexcept(boost::allocator_traits<Allocator>::is_always_equal::value ||
               boost::allocator_traits<Allocator>::propagate_on_container_move_assignment::value);
    unordered_flat_set& operator=(std::initializer_list<value_type>);
    allocator_type get_allocator() const noexcept;

    // iterators
    iterator       begin() noexcept;
    const_iterator begin() const noexcept;
    iterator       end() noexcept;
    const_iterator end() const noexcept;
    const_iterator cbegin() const noexcept;
    const_iterator cend() const noexcept;

    // capacity
    [[nodiscard]] bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;

    // modifiers
    template<class... Args> std::pair<iterator, bool> emplace(Args&&... args);
    template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
    std::pair<iterator, bool> insert(const value_type& obj);
    std::pair<iterator, bool> insert(value_type&& obj);
    template<class K> std::pair<iterator, bool> insert(K&& k);
    iterator insert(const_iterator hint, const value_type& obj);
    iterator insert(const_iterator hint, value_type&& obj);
    template<class K> iterator insert(const_iterator hint, K&& k);
    template<class InputIterator> void insert(InputIterator first, InputIterator last);
    void insert(std::initializer_list<value_type>);

    void      erase(iterator position);
    void      erase(const_iterator position);
    size_type erase(const key_type& k);
    template<class K> size_type erase(K&& k);
    iterator  erase(const_iterator first, const_iterator last);
    void      swap(unordered_flat_set& other)
      noexcept(boost::allocator_traits<Allocator>::is_always_equal::value ||
               boost::allocator_traits<Allocator>::propagate_on_container_swap::value);
    void      clear() noexcept;

    template<class H2, class P2>
      void merge(unordered_flat_set<Key, T, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_flat_set<Key, T, H2, P2, Allocator>&& source);

    // observers
    hasher hash_function() const;
    key_equal key_eq() const;

    // set operations
    iterator         find(const key_type& k);
    const_iterator   find(const key_type& k) const;
    template<class K>
      iterator       find(const K& k);
    template<class K>
      const_iterator find(const K& k) const;
    size_type        count(const key_type& k) const;
    template<class K>
      size_type      count(const K& k) const;
    bool             contains(const key_type& k) const;
    template<class K>
      bool           contains(const K& k) const;
    std::pair<iterator, iterator>               equal_range(const key_type& k);
    std::pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
    template<class K>
      std::pair<iterator, iterator>             equal_range(const K& k);
    template<class K>
      std::pair<const_iterator, const_iterator> equal_range(const K& k) const;

    // bucket interface
    size_type bucket_count() const noexcept;

    // hash policy
    float load_factor() const noexcept;
    float max_load_factor() const noexcept;
    void max_load_factor(float z);
    size_type max_load() const noexcept;
    void rehash(size_type n);
    void reserve(size_type n);
  };

  // Deduction Guides
  template<class InputIterator,
           class Hash = boost::hash<iter-value-type<InputIterator>>,
           class Pred = std::equal_to<iter-value-type<InputIterator>>,
           class Allocator = std::allocator<iter-value-type<InputIterator>>>
    unordered_flat_set(InputIterator, InputIterator, typename see below::size_type = see below,
                       Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_flat_set<iter-value-type<InputIterator>, Hash, Pred, Allocator>;

  template<class T, class Hash = boost::hash<T>, class Pred = std::equal_to<T>,
           class Allocator = std::allocator<T>>
    unordered_flat_set(std::initializer_list<T>, typename see below::size_type = see below,
                       Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_flat_set<T, Hash, Pred, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_flat_set(InputIterator, InputIterator, typename see below::size_type, Allocator)
      -> unordered_flat_set<iter-value-type<InputIterator>,
                            boost::hash<iter-value-type<InputIterator>>,
                            std::equal_to<iter-value-type<InputIterator>>, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_flat_set(InputIterator, InputIterator, Allocator)
      -> unordered_flat_set<iter-value-type<InputIterator>,
                            boost::hash<iter-value-type<InputIterator>>,
                            std::equal_to<iter-value-type<InputIterator>>, Allocator>;

  template<class InputIterator, class Hash, class Allocator>
    unordered_flat_set(InputIterator, InputIterator, typename see below::size_type, Hash,
                       Allocator)
      -> unordered_flat_set<iter-value-type<InputIterator>, Hash,
                            std::equal_to<iter-value-type<InputIterator>>, Allocator>;

  template<class T, class Allocator>
    unordered_flat_set(std::initializer_list<T>, typename see below::size_type, Allocator)
      -> unordered_flat_set<T, boost::hash<T>, std::equal_to<T>, Allocator>;

  template<class T, class Allocator>
    unordered_flat_set(std::initializer_list<T>, Allocator)
      -> unordered_flat_set<T, boost::hash<T>, std::equal_to<T>, Allocator>;

  template<class T, class Hash, class Allocator>
    unordered_flat_set(std::initializer_list<T>, typename see below::size_type, Hash, Allocator)
      -> unordered_flat_set<T, Hash, std::equal_to<T>, Allocator>;

  // Equality Comparisons
  template<class Key, class T, class Hash, class Pred, class Alloc>
    bool operator!=(const unordered_flat_set<Key, T, Hash, Pred, Alloc>& x,
                    const unordered_flat_set<Key, T, Hash, Pred, Alloc>& y);

  template<class Key, class T, class Hash, class Pred, class Alloc>
    bool operator!=(const unordered_flat_set<Key, T, Hash, Pred, Alloc>& x,
                    const unordered_flat_set<Key, T, Hash, Pred, Alloc>& y);

  // swap
  template<class Key, class T, class Hash, class Pred, class Alloc>
    void swap(unordered_flat_set<Key, T, Hash, Pred, Alloc>& x,
              unordered_flat_set<Key, T, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));

  template<class K, class T, class H, class P, class A, class Predicate>
    typename unordered_flat_set<K, T, H, P, A>::size_type
       erase_if(unordered_flat_set<K, T, H, P, A>& c, Predicate pred);
}

Description

Template Parameters

Key

Key must be MoveInsertable into the container and Erasable from the container.

Hash

A unary function object type that acts a hash function for a Key. It takes a single argument of type Key and returns a value of type std::size_t.

Pred

A binary function object that induces an equivalence relation on values of type Key. It takes two arguments of type Key and returns a value of type bool.

Allocator

An allocator whose value type is the same as the container’s value type. std::allocator_traits<Allocator>::pointer and std::allocator_traits<Allocator>::const_pointer must be convertible to/from value_type* and const value_type*, respectively.

The elements of the container are held into an internal bucket array. An element is inserted into a bucket determined by its hash code, but if the bucket is already occupied (a collision), an available one in the vicinity of the original position is used.

The size of the bucket array can be automatically increased by a call to insert/emplace, or as a result of calling rehash/reserve. The load factor of the container (number of elements divided by number of buckets) is never greater than max_load_factor(), except possibly for small sizes where the implementation may decide to allow for higher loads.

If hash_is_avalanching<Hash>::value is true, the hash function is used as-is; otherwise, a bit-mixing post-processing stage is added to increase the quality of hashing at the expense of extra computational cost.

Typedefs

typedef implementation-defined iterator;

A constant iterator whose value type is value_type.

The iterator category is at least a forward iterator.

Convertible to const_iterator.

typedef implementation-defined const_iterator;

A constant iterator whose value type is value_type.

The iterator category is at least a forward iterator.

Constructors

Default Constructor
unordered_flat_set();

Constructs an empty container using hasher() as the hash function, key_equal() as the key equality predicate and allocator_type() as the allocator.

Postconditions:

size() == 0
Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Bucket Count Constructor
explicit unordered_flat_set(size_type n,
                            const hasher& hf = hasher(),
                            const key_equal& eql = key_equal(),
                            const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate, and a as the allocator.

Postconditions:

size() == 0
Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Iterator Range Constructor
template<class InputIterator>
  unordered_flat_set(InputIterator f, InputIterator l,
                     size_type n = implementation-defined,
                     const hasher& hf = hasher(),
                     const key_equal& eql = key_equal(),
                     const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate and a as the allocator, and inserts the elements from [f, l) into it.

Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Copy Constructor
unordered_flat_set(unordered_flat_set const& other);

The copy constructor. Copies the contained elements, hash function, predicate and allocator.

If Allocator::select_on_container_copy_construction exists and has the right signature, the allocator will be constructed from its result.

Requires:

value_type is copy constructible
Move Constructor
unordered_flat_set(unordered_flat_set&& other);

The move constructor. The internal bucket array of other is transferred directly to the new container. The hash function, predicate and allocator are moved-constructed from other.

Iterator Range Constructor with Allocator
template<class InputIterator>
  unordered_flat_set(InputIterator f, InputIterator l, const allocator_type& a);

Constructs an empty container using a as the allocator, with the default hash function and key equality predicate and inserts the elements from [f, l) into it.

Requires:

hasher, key_equal need to be DefaultConstructible.
Allocator Constructor
explicit unordered_flat_set(Allocator const& a);

Constructs an empty container, using allocator a.

Copy Constructor with Allocator
unordered_flat_set(unordered_flat_set const& other, Allocator const& a);

Constructs a container, copying other's contained elements, hash function, and predicate, but using allocator a.

Move Constructor with Allocator
unordered_flat_set(unordered_flat_set&& other, Allocator const& a);

If a == other.get_allocator(), the elements of other are transferred directly to the new container; otherwise, elements are moved-constructed from those of other. The hash function and predicate are moved-constructed from other, and the allocator is copy-constructed from a.

Initializer List Constructor
unordered_flat_set(std::initializer_list<value_type> il,
              size_type n = implementation-defined
              const hasher& hf = hasher(),
              const key_equal& eql = key_equal(),
              const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate and a, and inserts the elements from il into it.

Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Bucket Count Constructor with Allocator
unordered_flat_set(size_type n, allocator_type const& a);

Constructs an empty container with at least n buckets, using hf as the hash function, the default hash function and key equality predicate and a as the allocator.

Postconditions:

size() == 0
Requires:

hasher and key_equal need to be DefaultConstructible.
Bucket Count Constructor with Hasher and Allocator
unordered_flat_set(size_type n, hasher const& hf, allocator_type const& a);

Constructs an empty container with at least n buckets, using hf as the hash function, the default key equality predicate and a as the allocator.

Postconditions:

size() == 0
Requires:

key_equal needs to be DefaultConstructible.
Iterator Range Constructor with Bucket Count and Allocator
template<class InputIterator>
  unordered_flat_set(InputIterator f, InputIterator l, size_type n, const allocator_type& a);

Constructs an empty container with at least n buckets, using a as the allocator and default hash function and key equality predicate, and inserts the elements from [f, l) into it.

Requires:

hasher, key_equal need to be DefaultConstructible.
Iterator Range Constructor with Bucket Count and Hasher
    template<class InputIterator>
      unordered_flat_set(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                         const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, a as the allocator, with the default key equality predicate, and inserts the elements from [f, l) into it.

Requires:

key_equal needs to be DefaultConstructible.
initializer_list Constructor with Allocator
unordered_flat_set(std::initializer_list<value_type> il, const allocator_type& a);

Constructs an empty container using a and default hash function and key equality predicate, and inserts the elements from il into it.

Requires:

hasher and key_equal need to be DefaultConstructible.
initializer_list Constructor with Bucket Count and Allocator
unordered_flat_set(std::initializer_list<value_type> il, size_type n, const allocator_type& a);

Constructs an empty container with at least n buckets, using a and default hash function and key equality predicate, and inserts the elements from il into it.

Requires:

hasher and key_equal need to be DefaultConstructible.
initializer_list Constructor with Bucket Count and Hasher and Allocator
unordered_flat_set(std::initializer_list<value_type> il, size_type n, const hasher& hf,
                   const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, a as the allocator and default key equality predicate,and inserts the elements from il into it.

Requires:

key_equal needs to be DefaultConstructible.

Destructor

~unordered_flat_set();
Note:

The destructor is applied to every element, and all memory is deallocated

Assignment

Copy Assignment
unordered_flat_set& operator=(unordered_flat_set const& other);

The assignment operator. Destroys previously existing elements, copy-assigns the hash function and predicate from other, copy-assigns the allocator from other if Alloc::propagate_on_container_copy_assignment exists and Alloc::propagate_on_container_copy_assignment::value is true, and finally inserts copies of the elements of other.

Requires:

value_type is CopyInsertable
Move Assignment
unordered_flat_set& operator=(unordered_flat_set&& other)
  noexcept(boost::allocator_traits<Allocator>::is_always_equal::value ||
           boost::allocator_traits<Allocator>::propagate_on_container_move_assignment::value);

The move assignment operator. Destroys previously existing elements, swaps the hash function and predicate from other, and move-assigns the allocator from other if Alloc::propagate_on_container_move_assignment exists and Alloc::propagate_on_container_move_assignment::value is true. If at this point the allocator is equal to other.get_allocator(), the internal bucket array of other is transferred directly to the new container; otherwise, inserts move-constructed copies of the elements of other.

Initializer List Assignment
unordered_flat_set& operator=(std::initializer_list<value_type> il);

Assign from values in initializer list. All previously existing elements are destroyed.

Requires:

value_type is CopyInsertable

Iterators

begin
iterator begin() noexcept;
const_iterator begin() const noexcept;
Returns:

An iterator referring to the first element of the container, or if the container is empty the past-the-end value for the container.
Complexity:

O(bucket_count())
end
iterator end() noexcept;
const_iterator end() const noexcept;
Returns:

An iterator which refers to the past-the-end value for the container.
cbegin
const_iterator cbegin() const noexcept;
Returns:

A const_iterator referring to the first element of the container, or if the container is empty the past-the-end value for the container.
Complexity:

O(bucket_count())
cend
const_iterator cend() const noexcept;
Returns:

A const_iterator which refers to the past-the-end value for the container.

Size and Capacity

empty
[[nodiscard]] bool empty() const noexcept;
Returns:

size() == 0
size
size_type size() const noexcept;
Returns:

std::distance(begin(), end())
max_size
size_type max_size() const noexcept;
Returns:

size() of the largest possible container.

Modifiers

emplace
template<class... Args> std::pair<iterator, bool> emplace(Args&&... args);

Inserts an object, constructed with the arguments args, in the container if and only if there is no element in the container with an equivalent key.

Requires:

value_type is constructible from args.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.
emplace_hint
    template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);

Inserts an object, constructed with the arguments args, in the container if and only if there is no element in the container with an equivalent key.

position is a suggestion to where the element should be inserted. This implementation ignores it.

Requires:

value_type is constructible from args.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.
Copy Insert
std::pair<iterator, bool> insert(const value_type& obj);

Inserts obj in the container if and only if there is no element in the container with an equivalent key.

Requires:

value_type is CopyInsertable.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.
Move Insert
std::pair<iterator, bool> insert(value_type&& obj);

Inserts obj in the container if and only if there is no element in the container with an equivalent key.

Requires:

value_type is MoveInsertable.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.
Transparent Insert
template<class K> std::pair<iterator, bool> insert(K&& k);

Inserts an element constructed from std::forward<K>(k) in the container if and only if there is no element in the container with an equivalent key.

Requires:

value_type is EmplaceConstructible from k.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.

This overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
Copy Insert with Hint
iterator insert(const_iterator hint, const value_type& obj);

Inserts obj in the container if and only if there is no element in the container with an equivalent key.

hint is a suggestion to where the element should be inserted. This implementation ignores it.

Requires:

value_type is CopyInsertable.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.
Move Insert with Hint
iterator insert(const_iterator hint, value_type&& obj);

Inserts obj in the container if and only if there is no element in the container with an equivalent key.

hint is a suggestion to where the element should be inserted. This implementation ignores it.

Requires:

value_type is MoveInsertable.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.
Transparent Insert with Hint
template<class K> std::pair<iterator, bool> insert(const_iterator hint, K&& k);

Inserts an element constructed from std::forward<K>(k) in the container if and only if there is no element in the container with an equivalent key.

hint is a suggestion to where the element should be inserted. This implementation ignores it.

Requires:

value_type is EmplaceConstructible from k.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.

This overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
Insert Iterator Range
template<class InputIterator> void insert(InputIterator first, InputIterator last);

Inserts a range of elements into the container. Elements are inserted if and only if there is no element in the container with an equivalent key.

Requires:

value_type is EmplaceConstructible into the container from *first.
Throws:

When inserting a single element, if an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.
Insert Initializer List
void insert(std::initializer_list<value_type>);

Inserts a range of elements into the container. Elements are inserted if and only if there is no element in the container with an equivalent key.

Requires:

value_type is EmplaceConstructible into the container from *first.
Throws:

When inserting a single element, if an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load.
Erase by Position
void erase(iterator position);
void erase(const_iterator position);

Erase the element pointed to by position.

Throws:

Nothing.
Erase by Key
size_type erase(const key_type& k);
template<class K> size_type erase(K&& k);

Erase all elements with key equivalent to k.

Returns:

The number of elements erased.
Throws:

Only throws an exception if it is thrown by hasher or key_equal.
Notes:

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
Erase Range
iterator erase(const_iterator first, const_iterator last);

Erases the elements in the range from first to last.

Returns:

The iterator following the erased elements - i.e. last.
Throws:

Nothing in this implementation (neither the hasher nor the key_equal objects are called).
swap
void swap(unordered_flat_set& other)
  noexcept(boost::allocator_traits<Allocator>::is_always_equal::value ||
           boost::allocator_traits<Allocator>::propagate_on_container_swap::value);

Swaps the contents of the container with the parameter.

If Allocator::propagate_on_container_swap is declared and Allocator::propagate_on_container_swap::value is true then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.

Throws:

Nothing unless key_equal or hasher throw on swapping.
clear
void clear() noexcept;

Erases all elements in the container.

Postconditions:

size() == 0, max_load() >= max_load_factor() * bucket_count()
merge
template<class H2, class P2>
  void merge(unordered_flat_set<Key, T, H2, P2, Allocator>& source);
template<class H2, class P2>
  void merge(unordered_flat_set<Key, T, H2, P2, Allocator>&& source);

Move-inserts all the elements from source whose key is not already present in *this, and erases them from source.

Observers

get_allocator
allocator_type get_allocator() const noexcept;
Returns:

The container’s allocator.
hash_function
hasher hash_function() const;
Returns:

The container’s hash function.
key_eq
key_equal key_eq() const;
Returns:

The container’s key equality predicate

Lookup

find
iterator         find(const key_type& k);
const_iterator   find(const key_type& k) const;
template<class K>
  iterator       find(const K& k);
Returns:

An iterator pointing to an element with key equivalent to k, or end() if no such element exists.
Notes:

The template <typename K> overloads only participate in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
count
size_type        count(const key_type& k) const;
template<class K>
  size_type      count(const K& k) const;
Returns:

The number of elements with key equivalent to k.
Notes:

The template <typename K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
contains
bool             contains(const key_type& k) const;
template<class K>
  bool           contains(const K& k) const;
Returns:

A boolean indicating whether or not there is an element with key equal to key in the container
Notes:

The template <typename K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
equal_range
std::pair<iterator, iterator>               equal_range(const key_type& k);
std::pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
template<class K>
  std::pair<iterator, iterator>             equal_range(const K& k);
template<class K>
  std::pair<const_iterator, const_iterator> equal_range(const K& k) const;
Returns:

A range containing all elements with key equivalent to k. If the container doesn’t contain any such elements, returns std::make_pair(b.end(), b.end()).
Notes:

The template <typename K> overloads only participate in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.

Bucket Interface

bucket_count
size_type bucket_count() const noexcept;
Returns:

The size of the bucket array.

Hash Policy

load_factor
float load_factor() const noexcept;
Returns:

static_cast<float>(size())/static_cast<float>(bucket_count()), or 0 if bucket_count() == 0.
max_load_factor
float max_load_factor() const noexcept;
Returns:

Returns the container’s maximum load factor.
Set max_load_factor
void max_load_factor(float z);
Effects:

Does nothing, as the user is not allowed to change this parameter. Kept for compatibility with boost::unordered_set.
max_load
size_type max_load() const noexcept;
Returns:

The maximum number of elements the container can hold without rehashing, assuming that no further elements will be erased.
Note:

After construction, rehash or clearance, the container’s maximum load is at least max_load_factor() * bucket_count(). This number may decrease on erasure under high-load conditions.
rehash
void rehash(size_type n);

Changes if necessary the size of the bucket array so that there are at least n buckets, and so that the load factor is less than or equal to the maximum load factor. When applicable, this will either grow or shrink the bucket_count() associated with the container.

When size() == 0, rehash(0) will deallocate the underlying buckets array.

Invalidates iterators, pointers and references, and changes the order of elements.

Throws:

The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function.
reserve
void reserve(size_type n);

Equivalent to a.rehash(ceil(n / a.max_load_factor())).

Similar to rehash, this function can be used to grow or shrink the number of buckets in the container.

Invalidates iterators, pointers and references, and changes the order of elements.

Throws:

The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function.

Deduction Guides

A deduction guide will not participate in overload resolution if any of the following are true:

  • It has an InputIterator template parameter and a type that does not qualify as an input iterator is deduced for that parameter.

  • It has an Allocator template parameter and a type that does not qualify as an allocator is deduced for that parameter.

  • It has a Hash template parameter and an integral type or a type that qualifies as an allocator is deduced for that parameter.

  • It has a Pred template parameter and a type that qualifies as an allocator is deduced for that parameter.

A size_­type parameter type in a deduction guide refers to the size_­type member type of the container type deduced by the deduction guide. Its default value coincides with the default value of the constructor selected.

iter-value-type
template<class InputIterator>
  using iter-value-type =
    typename std::iterator_traits<InputIterator>::value_type; // exposition only

Equality Comparisons

operator==
template<class Key, class T, class Hash, class Pred, class Alloc>
  bool operator==(const unordered_flat_set<Key, T, Hash, Pred, Alloc>& x,
                  const unordered_flat_set<Key, T, Hash, Pred, Alloc>& y);

Return true if x.size() == y.size() and for every element in x, there is an element in y with the same key, with an equal value (using operator== to compare the value types).

Notes:

Behavior is undefined if the two containers don’t have equivalent equality predicates.
operator!=
template<class Key, class T, class Hash, class Pred, class Alloc>
  bool operator!=(const unordered_flat_set<Key, T, Hash, Pred, Alloc>& x,
                  const unordered_flat_set<Key, T, Hash, Pred, Alloc>& y);

Return false if x.size() == y.size() and for every element in x, there is an element in y with the same key, with an equal value (using operator== to compare the value types).

Notes:

Behavior is undefined if the two containers don’t have equivalent equality predicates.

Swap

template<class Key, class T, class Hash, class Pred, class Alloc>
  void swap(unordered_flat_set<Key, T, Hash, Pred, Alloc>& x,
            unordered_flat_set<Key, T, Hash, Pred, Alloc>& y)
    noexcept(noexcept(x.swap(y)));

Swaps the contents of x and y.

If Allocator::propagate_on_container_swap is declared and Allocator::propagate_on_container_swap::value is true then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.

Effects:

x.swap(y)
Throws:

Nothing unless key_equal or hasher throw on swapping.

erase_if

template<class K, class T, class H, class P, class A, class Predicate>
  typename unordered_flat_set<K, T, H, P, A>::size_type
    erase_if(unordered_flat_set<K, T, H, P, A>& c, Predicate pred);

Traverses the container c and removes all elements for which the supplied predicate returns true.

Returns:

The number of erased elements.
Notes:

Equivalent to:

auto original_size = c.size();
for (auto i = c.begin(), last = c.end(); i != last; ) {
  if (pred(*i)) {
    i = c.erase(i);
  } else {
    ++i;
  }
}
return original_size - c.size();

Class template unordered_node_map

boost::unordered_node_map — A node-based, open-addressing unordered associative container that associates unique keys with another value.

boost::unordered_node_map uses an open-addressing layout like boost::unordered_flat_map, but, being node-based, it provides pointer/iterator stability and node handling functionalities. Its performance lies between those of boost::unordered_map and boost::unordered_flat_map.

As a result of its using open addressing, the interface of boost::unordered_node_map deviates in a number of aspects from that of boost::unordered_map/std::unordered_map:

  • begin() is not constant-time.

  • erase(iterator) returns void.

  • There is no API for bucket handling (except bucket_count).

  • The maximum load factor of the container is managed internally and can’t be set by the user.

Other than this, boost::unordered_node_map is mostly a drop-in replacement of standard unordered associative containers.

Synopsis

// #include <boost/unordered/unordered_node_map.hpp>

namespace boost {
  template<class Key,
           class T,
           class Hash = boost::hash<Key>,
           class Pred = std::equal_to<Key>,
           class Allocator = std::allocator<std::pair<const Key, T>>>
  class unordered_node_map {
  public:
    // types
    using key_type             = Key;
    using mapped_type          = T;
    using value_type           = std::pair<const Key, T>;
    using init_type            = std::pair<
                                   typename std::remove_const<Key>::type,
                                   typename std::remove_const<T>::type
                                 >;
    using hasher               = Hash;
    using key_equal            = Pred;
    using allocator_type       = Allocator;
    using pointer              = typename std::allocator_traits<Allocator>::pointer;
    using const_pointer        = typename std::allocator_traits<Allocator>::const_pointer;
    using reference            = value_type&;
    using const_reference      = const value_type&;
    using size_type            = std::size_t;
    using difference_type      = std::ptrdiff_t;

    using iterator             = implementation-defined;
    using const_iterator       = implementation-defined;

    using node_type            = implementation-defined;
    using insert_return_type   = implementation-defined;

    // construct/copy/destroy
    unordered_node_map();
    explicit unordered_node_map(size_type n,
                                const hasher& hf = hasher(),
                                const key_equal& eql = key_equal(),
                                const allocator_type& a = allocator_type());
    template<class InputIterator>
      unordered_node_map(InputIterator f, InputIterator l,
                         size_type n = implementation-defined,
                         const hasher& hf = hasher(),
                         const key_equal& eql = key_equal(),
                         const allocator_type& a = allocator_type());
    unordered_node_map(const unordered_node_map& other);
    unordered_node_map(unordered_node_map&& other);
    template<class InputIterator>
      unordered_node_map(InputIterator f, InputIterator l, const allocator_type& a);
    explicit unordered_node_map(const Allocator& a);
    unordered_node_map(const unordered_node_map& other, const Allocator& a);
    unordered_node_map(unordered_node_map&& other, const Allocator& a);
    unordered_node_map(std::initializer_list<value_type> il,
                       size_type n = implementation-defined
                       const hasher& hf = hasher(),
                       const key_equal& eql = key_equal(),
                       const allocator_type& a = allocator_type());
    unordered_node_map(size_type n, const allocator_type& a);
    unordered_node_map(size_type n, const hasher& hf, const allocator_type& a);
    template<class InputIterator>
      unordered_node_map(InputIterator f, InputIterator l, size_type n, const allocator_type& a);
    template<class InputIterator>
      unordered_node_map(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                         const allocator_type& a);
    unordered_node_map(std::initializer_list<value_type> il, const allocator_type& a);
    unordered_node_map(std::initializer_list<value_type> il, size_type n,
                       const allocator_type& a);
    unordered_node_map(std::initializer_list<value_type> il, size_type n, const hasher& hf,
                       const allocator_type& a);
    ~unordered_node_map();
    unordered_node_map& operator=(const unordered_node_map& other);
    unordered_node_map& operator=(unordered_node_map&& other)
      noexcept(boost::allocator_traits<Allocator>::is_always_equal::value ||
              boost::allocator_traits<Allocator>::propagate_on_container_move_assignment::value);
    unordered_node_map& operator=(std::initializer_list<value_type>);
    allocator_type get_allocator() const noexcept;

    // iterators
    iterator       begin() noexcept;
    const_iterator begin() const noexcept;
    iterator       end() noexcept;
    const_iterator end() const noexcept;
    const_iterator cbegin() const noexcept;
    const_iterator cend() const noexcept;

    // capacity
    [[nodiscard]] bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;

    // modifiers
    template<class... Args> std::pair<iterator, bool> emplace(Args&&... args);
    template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
    std::pair<iterator, bool> insert(const value_type& obj);
    std::pair<iterator, bool> insert(const init_type& obj);
    std::pair<iterator, bool> insert(value_type&& obj);
    std::pair<iterator, bool> insert(init_type&& obj);
    iterator       insert(const_iterator hint, const value_type& obj);
    iterator       insert(const_iterator hint, const init_type& obj);
    iterator       insert(const_iterator hint, value_type&& obj);
    iterator       insert(const_iterator hint, init_type&& obj);
    template<class InputIterator> void insert(InputIterator first, InputIterator last);
    void insert(std::initializer_list<value_type>);
    insert_return_type insert(node_type&& nh);
    iterator insert(const_iterator hint, node_type&& nh);

    template<class... Args>
      std::pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
    template<class... Args>
      std::pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
    template<class K, class... Args>
      std::pair<iterator, bool> try_emplace(K&& k, Args&&... args);
    template<class... Args>
      iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
    template<class... Args>
      iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);
    template<class K, class... Args>
      iterator try_emplace(const_iterator hint, K&& k, Args&&... args);
    template<class M>
      std::pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
    template<class M>
      std::pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
    template<class K, class M>
      std::pair<iterator, bool> insert_or_assign(K&& k, M&& obj);
    template<class M>
      iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
    template<class M>
      iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);
    template<class K, class M>
      iterator insert_or_assign(const_iterator hint, K&& k, M&& obj);

    void      erase(iterator position);
    void      erase(const_iterator position);
    size_type erase(const key_type& k);
    template<class K> size_type erase(K&& k);
    iterator  erase(const_iterator first, const_iterator last);
    void      swap(unordered_node_map& other)
      noexcept(boost::allocator_traits<Allocator>::is_always_equal::value ||
               boost::allocator_traits<Allocator>::propagate_on_container_swap::value);
    node_type extract(const_iterator position);
    node_type extract(const key_type& key);
    template<class K> node_type extract(K&& key);
    void      clear() noexcept;

    template<class H2, class P2>
      void merge(unordered_node_map<Key, T, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_node_map<Key, T, H2, P2, Allocator>&& source);

    // observers
    hasher hash_function() const;
    key_equal key_eq() const;

    // map operations
    iterator         find(const key_type& k);
    const_iterator   find(const key_type& k) const;
    template<class K>
      iterator       find(const K& k);
    template<class K>
      const_iterator find(const K& k) const;
    size_type        count(const key_type& k) const;
    template<class K>
      size_type      count(const K& k) const;
    bool             contains(const key_type& k) const;
    template<class K>
      bool           contains(const K& k) const;
    std::pair<iterator, iterator>               equal_range(const key_type& k);
    std::pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
    template<class K>
      std::pair<iterator, iterator>             equal_range(const K& k);
    template<class K>
      std::pair<const_iterator, const_iterator> equal_range(const K& k) const;

    // element access
    mapped_type& operator[](const key_type& k);
    mapped_type& operator[](key_type&& k);
    template<class K> mapped_type& operator[](K&& k);
    mapped_type& at(const key_type& k);
    const mapped_type& at(const key_type& k) const;
    template<class K> mapped_type& at(const K& k);
    template<class K> const mapped_type& at(const K& k) const;

    // bucket interface
    size_type bucket_count() const noexcept;

    // hash policy
    float load_factor() const noexcept;
    float max_load_factor() const noexcept;
    void max_load_factor(float z);
    size_type max_load() const noexcept;
    void rehash(size_type n);
    void reserve(size_type n);
  };

  // Deduction Guides
  template<class InputIterator,
           class Hash = boost::hash<iter-key-type<InputIterator>>,
           class Pred = std::equal_to<iter-key-type<InputIterator>>,
           class Allocator = std::allocator<iter-to-alloc-type<InputIterator>>>
    unordered_node_map(InputIterator, InputIterator, typename see below::size_type = see below,
                       Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_node_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Hash,
                            Pred, Allocator>;

  template<class Key, class T, class Hash = boost::hash<Key>,
           class Pred = std::equal_to<Key>,
           class Allocator = std::allocator<std::pair<const Key, T>>>
    unordered_node_map(std::initializer_list<std::pair<Key, T>>,
                       typename see below::size_type = see below, Hash = Hash(),
                       Pred = Pred(), Allocator = Allocator())
      -> unordered_node_map<Key, T, Hash, Pred, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_node_map(InputIterator, InputIterator, typename see below::size_type, Allocator)
      -> unordered_node_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
                            boost::hash<iter-key-type<InputIterator>>,
                            std::equal_to<iter-key-type<InputIterator>>, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_node_map(InputIterator, InputIterator, Allocator)
      -> unordered_node_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
                            boost::hash<iter-key-type<InputIterator>>,
                            std::equal_to<iter-key-type<InputIterator>>, Allocator>;

  template<class InputIterator, class Hash, class Allocator>
    unordered_node_map(InputIterator, InputIterator, typename see below::size_type, Hash,
                       Allocator)
      -> unordered_node_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Hash,
                            std::equal_to<iter-key-type<InputIterator>>, Allocator>;

  template<class Key, class T, class Allocator>
    unordered_node_map(std::initializer_list<std::pair<Key, T>>, typename see below::size_type,
                       Allocator)
      -> unordered_node_map<Key, T, boost::hash<Key>, std::equal_to<Key>, Allocator>;

  template<class Key, class T, class Allocator>
    unordered_node_map(std::initializer_list<std::pair<Key, T>>, Allocator)
      -> unordered_node_map<Key, T, boost::hash<Key>, std::equal_to<Key>, Allocator>;

  template<class Key, class T, class Hash, class Allocator>
    unordered_node_map(std::initializer_list<std::pair<Key, T>>, typename see below::size_type,
                       Hash, Allocator)
      -> unordered_node_map<Key, T, Hash, std::equal_to<Key>, Allocator>;

  // Equality Comparisons
  template<class Key, class T, class Hash, class Pred, class Alloc>
    bool operator==(const unordered_node_map<Key, T, Hash, Pred, Alloc>& x,
                    const unordered_node_map<Key, T, Hash, Pred, Alloc>& y);

  template<class Key, class T, class Hash, class Pred, class Alloc>
    bool operator!=(const unordered_node_map<Key, T, Hash, Pred, Alloc>& x,
                    const unordered_node_map<Key, T, Hash, Pred, Alloc>& y);

  // swap
  template<class Key, class T, class Hash, class Pred, class Alloc>
    void swap(unordered_node_map<Key, T, Hash, Pred, Alloc>& x,
              unordered_node_map<Key, T, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));

  template<class K, class T, class H, class P, class A, class Predicate>
    typename unordered_node_map<K, T, H, P, A>::size_type
       erase_if(unordered_node_map<K, T, H, P, A>& c, Predicate pred);
}

Description

Template Parameters

Key

std::pair<const Key, T> must be EmplaceConstructible into the container from any std::pair object convertible to it, and it also must be Erasable from the container.

T

Hash

A unary function object type that acts a hash function for a Key. It takes a single argument of type Key and returns a value of type std::size_t.

Pred

A binary function object that induces an equivalence relation on values of type Key. It takes two arguments of type Key and returns a value of type bool.

Allocator

An allocator whose value type is the same as the container’s value type. std::allocator_traits<Allocator>::pointer and std::allocator_traits<Allocator>::const_pointer must be convertible to/from value_type* and const value_type*, respectively.

The element nodes of the container are held into an internal bucket array. A node is inserted into a bucket determined by the hash code of its element, but if the bucket is already occupied (a collision), an available one in the vicinity of the original position is used.

The size of the bucket array can be automatically increased by a call to insert/emplace, or as a result of calling rehash/reserve. The load factor of the container (number of elements divided by number of buckets) is never greater than max_load_factor(), except possibly for small sizes where the implementation may decide to allow for higher loads.

If hash_is_avalanching<Hash>::value is true, the hash function is used as-is; otherwise, a bit-mixing post-processing stage is added to increase the quality of hashing at the expense of extra computational cost.

Typedefs

typedef implementation-defined iterator;

An iterator whose value type is value_type.

The iterator category is at least a forward iterator.

Convertible to const_iterator.

typedef implementation-defined const_iterator;

A constant iterator whose value type is value_type.

The iterator category is at least a forward iterator.

typedef implementation-defined node_type;

A class for holding extracted container elements, modelling NodeHandle.

typedef implementation-defined insert_return_type;

A specialization of an internal class template:

template<class Iterator, class NodeType>
struct insert_return_type // name is exposition only
{
  Iterator position;
  bool     inserted;
  NodeType node;
};

with Iterator = iterator and NodeType = node_type.

Constructors

Default Constructor
unordered_node_map();

Constructs an empty container using hasher() as the hash function, key_equal() as the key equality predicate and allocator_type() as the allocator.

Postconditions:

size() == 0
Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Bucket Count Constructor
explicit unordered_node_map(size_type n,
                            const hasher& hf = hasher(),
                            const key_equal& eql = key_equal(),
                            const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate, and a as the allocator.

Postconditions:

size() == 0
Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Iterator Range Constructor
template<class InputIterator>
  unordered_node_map(InputIterator f, InputIterator l,
                     size_type n = implementation-defined,
                     const hasher& hf = hasher(),
                     const key_equal& eql = key_equal(),
                     const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate and a as the allocator, and inserts the elements from [f, l) into it.

Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Copy Constructor
unordered_node_map(unordered_node_map const& other);

The copy constructor. Copies the contained elements, hash function, predicate and allocator.

If Allocator::select_on_container_copy_construction exists and has the right signature, the allocator will be constructed from its result.

Move Constructor
unordered_node_map(unordered_node_map&& other);

The move constructor. The internal bucket array of other is transferred directly to the new container. The hash function, predicate and allocator are moved-constructed from other.

Iterator Range Constructor with Allocator
template<class InputIterator>
  unordered_node_map(InputIterator f, InputIterator l, const allocator_type& a);

Constructs an empty container using a as the allocator, with the default hash function and key equality predicate and inserts the elements from [f, l) into it.

Requires:

hasher, key_equal need to be DefaultConstructible.
Allocator Constructor
explicit unordered_node_map(Allocator const& a);

Constructs an empty container, using allocator a.

Copy Constructor with Allocator
unordered_node_map(unordered_node_map const& other, Allocator const& a);

Constructs a container, copying other's contained elements, hash function, and predicate, but using allocator a.

Move Constructor with Allocator
unordered_node_map(unordered_node_map&& other, Allocator const& a);

If a == other.get_allocator(), the element nodes of other are transferred directly to the new container; otherwise, elements are moved-constructed from those of other. The hash function and predicate are moved-constructed from other, and the allocator is copy-constructed from a.

Initializer List Constructor
unordered_node_map(std::initializer_list<value_type> il,
              size_type n = implementation-defined
              const hasher& hf = hasher(),
              const key_equal& eql = key_equal(),
              const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate and a, and inserts the elements from il into it.

Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Bucket Count Constructor with Allocator
unordered_node_map(size_type n, allocator_type const& a);

Constructs an empty container with at least n buckets, using hf as the hash function, the default hash function and key equality predicate and a as the allocator.

Postconditions:

size() == 0
Requires:

hasher and key_equal need to be DefaultConstructible.
Bucket Count Constructor with Hasher and Allocator
unordered_node_map(size_type n, hasher const& hf, allocator_type const& a);

Constructs an empty container with at least n buckets, using hf as the hash function, the default key equality predicate and a as the allocator.

Postconditions:

size() == 0
Requires:

key_equal needs to be DefaultConstructible.
Iterator Range Constructor with Bucket Count and Allocator
template<class InputIterator>
  unordered_node_map(InputIterator f, InputIterator l, size_type n, const allocator_type& a);

Constructs an empty container with at least n buckets, using a as the allocator and default hash function and key equality predicate, and inserts the elements from [f, l) into it.

Requires:

hasher, key_equal need to be DefaultConstructible.
Iterator Range Constructor with Bucket Count and Hasher
    template<class InputIterator>
      unordered_node_map(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                         const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, a as the allocator, with the default key equality predicate, and inserts the elements from [f, l) into it.

Requires:

key_equal needs to be DefaultConstructible.
initializer_list Constructor with Allocator
unordered_node_map(std::initializer_list<value_type> il, const allocator_type& a);

Constructs an empty container using a and default hash function and key equality predicate, and inserts the elements from il into it.

Requires:

hasher and key_equal need to be DefaultConstructible.
initializer_list Constructor with Bucket Count and Allocator
unordered_node_map(std::initializer_list<value_type> il, size_type n, const allocator_type& a);

Constructs an empty container with at least n buckets, using a and default hash function and key equality predicate, and inserts the elements from il into it.

Requires:

hasher and key_equal need to be DefaultConstructible.
initializer_list Constructor with Bucket Count and Hasher and Allocator
unordered_node_map(std::initializer_list<value_type> il, size_type n, const hasher& hf,
                   const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, a as the allocator and default key equality predicate,and inserts the elements from il into it.

Requires:

key_equal needs to be DefaultConstructible.

Destructor

~unordered_node_map();
Note:

The destructor is applied to every element, and all memory is deallocated

Assignment

Copy Assignment
unordered_node_map& operator=(unordered_node_map const& other);

The assignment operator. Destroys previously existing elements, copy-assigns the hash function and predicate from other, copy-assigns the allocator from other if Alloc::propagate_on_container_copy_assignment exists and Alloc::propagate_on_container_copy_assignment::value is true, and finally inserts copies of the elements of other.

Requires:

value_type is CopyInsertable
Move Assignment
unordered_node_map& operator=(unordered_node_map&& other)
  noexcept(boost::allocator_traits<Allocator>::is_always_equal::value ||
           boost::allocator_traits<Allocator>::propagate_on_container_move_assignment::value);

The move assignment operator. Destroys previously existing elements, swaps the hash function and predicate from other, and move-assigns the allocator from other if Alloc::propagate_on_container_move_assignment exists and Alloc::propagate_on_container_move_assignment::value is true. If at this point the allocator is equal to other.get_allocator(), the internal bucket array of other is transferred directly to the new container; otherwise, inserts move-constructed copies of the elements of other.

Initializer List Assignment
unordered_node_map& operator=(std::initializer_list<value_type> il);

Assign from values in initializer list. All previously existing elements are destroyed.

Requires:

value_type is CopyInsertable

Iterators

begin
iterator begin() noexcept;
const_iterator begin() const noexcept;
Returns:

An iterator referring to the first element of the container, or if the container is empty the past-the-end value for the container.
Complexity:

O(bucket_count())
end
iterator end() noexcept;
const_iterator end() const noexcept;
Returns:

An iterator which refers to the past-the-end value for the container.
cbegin
const_iterator cbegin() const noexcept;
Returns:

A const_iterator referring to the first element of the container, or if the container is empty the past-the-end value for the container.
Complexity:

O(bucket_count())
cend
const_iterator cend() const noexcept;
Returns:

A const_iterator which refers to the past-the-end value for the container.

Size and Capacity

empty
[[nodiscard]] bool empty() const noexcept;
Returns:

size() == 0
size
size_type size() const noexcept;
Returns:

std::distance(begin(), end())
max_size
size_type max_size() const noexcept;
Returns:

size() of the largest possible container.

Modifiers

emplace
template<class... Args> std::pair<iterator, bool> emplace(Args&&... args);

Inserts an object, constructed with the arguments args, in the container if and only if there is no element in the container with an equivalent key.

Requires:

value_type is constructible from args.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load to be greater than the maximum load.
emplace_hint
    template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);

Inserts an object, constructed with the arguments args, in the container if and only if there is no element in the container with an equivalent key.

position is a suggestion to where the element should be inserted. This implementation ignores it.

Requires:

value_type is constructible from args.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load to be greater than the maximum load.
Copy Insert
std::pair<iterator, bool> insert(const value_type& obj);
std::pair<iterator, bool> insert(const init_type& obj);

Inserts obj in the container if and only if there is no element in the container with an equivalent key.

Requires:

value_type is CopyInsertable.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load to be greater than the maximum load.

A call of the form insert(x), where x is equally convertible to both const value_type& and const init_type&, is not ambiguous and selects the init_type overload.
Move Insert
std::pair<iterator, bool> insert(value_type&& obj);
std::pair<iterator, bool> insert(init_type&& obj);

Inserts obj in the container if and only if there is no element in the container with an equivalent key.

Requires:

value_type is MoveInsertable.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load to be greater than the maximum load.

A call of the form insert(x), where x is equally convertible to both value_type&& and init_type&&, is not ambiguous and selects the init_type overload.
Copy Insert with Hint
iterator insert(const_iterator hint, const value_type& obj);
iterator insert(const_iterator hint, const init_type& obj);

Inserts obj in the container if and only if there is no element in the container with an equivalent key.

hint is a suggestion to where the element should be inserted. This implementation ignores it.

Requires:

value_type is CopyInsertable.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load to be greater than the maximum load.

A call of the form insert(hint, x), where x is equally convertible to both const value_type& and const init_type&, is not ambiguous and selects the init_type overload.
Move Insert with Hint
iterator insert(const_iterator hint, value_type&& obj);
iterator insert(const_iterator hint, init_type&& obj);

Inserts obj in the container if and only if there is no element in the container with an equivalent key.

hint is a suggestion to where the element should be inserted. This implementation ignores it.

Requires:

value_type is MoveInsertable.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load to be greater than the maximum load.

A call of the form insert(hint, x), where x is equally convertible to both value_type&& and init_type&&, is not ambiguous and selects the init_type overload.
Insert Iterator Range
template<class InputIterator> void insert(InputIterator first, InputIterator last);

Inserts a range of elements into the container. Elements are inserted if and only if there is no element in the container with an equivalent key.

Requires:

value_type is EmplaceConstructible into the container from *first.
Throws:

When inserting a single element, if an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load to be greater than the maximum load.
Insert Initializer List
void insert(std::initializer_list<value_type>);

Inserts a range of elements into the container. Elements are inserted if and only if there is no element in the container with an equivalent key.

Requires:

value_type is EmplaceConstructible into the container from *first.
Throws:

When inserting a single element, if an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load to be greater than the maximum load.
Insert Node
insert_return_type insert(node_type&& nh);

If nh is not empty, inserts the associated element in the container if and only if there is no element in the container with a key equivalent to nh.key(). nh is empty when the function returns.

Returns:

An insert_return_type object constructed from position, inserted and node:

  • If nh is empty, inserted is false, position is end(), and node is empty.

  • Otherwise if the insertion took place, inserted is true, position points to the inserted element, and node is empty.

  • If the insertion failed, inserted is false, node has the previous value of nh, and position points to an element with a key equivalent to nh.key().
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Behavior is undefined if nh is not empty and the allocators of nh and the container are not equal.
Insert Node with Hint
iterator insert(const_iterator hint, node_type&& nh);

If nh is not empty, inserts the associated element in the container if and only if there is no element in the container with a key equivalent to nh.key(). nh becomes empty if insertion took place, otherwise it is not changed.

hint is a suggestion to where the element should be inserted. This implementation ignores it.

Returns:

The iterator returned is end() if nh is empty. If insertion took place, then the iterator points to the newly inserted element; otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Behavior is undefined if nh is not empty and the allocators of nh and the container are not equal.
try_emplace
template<class... Args>
  std::pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
template<class... Args>
  std::pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
template<class K, class... Args>
  std::pair<iterator, bool> try_emplace(K&& k, Args&&... args);

Inserts a new node into the container if there is no existing element with key k contained within it.

If there is an existing element with key k this function does nothing.

Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

This function is similiar to emplace, with the difference that no value_type is constructed if there is an element with an equivalent key; otherwise, the construction is of the form:

// first two overloads
value_type(std::piecewise_construct,
           std::forward_as_tuple(boost::forward<Key>(k)),
           std::forward_as_tuple(boost::forward<Args>(args)...))

// third overload
value_type(std::piecewise_construct,
           std::forward_as_tuple(boost::forward<K>(k)),
           std::forward_as_tuple(boost::forward<Args>(args)...))

unlike emplace, which simply forwards all arguments to value_type's constructor.

Can invalidate iterators, but only if the insert causes the load to be greater than the maximum load.

The template <class K, class…​ Args> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
try_emplace with Hint
template<class... Args>
  iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
template<class... Args>
  iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);
template<class K, class... Args>
  iterator try_emplace(const_iterator hint, K&& k, Args&&... args);

Inserts a new node into the container if there is no existing element with key k contained within it.

If there is an existing element with key k this function does nothing.

hint is a suggestion to where the element should be inserted. This implementation ignores it.

Returns:

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

This function is similiar to emplace_hint, with the difference that no value_type is constructed if there is an element with an equivalent key; otherwise, the construction is of the form:

// first two overloads
value_type(std::piecewise_construct,
           std::forward_as_tuple(boost::forward<Key>(k)),
           std::forward_as_tuple(boost::forward<Args>(args)...))

// third overload
value_type(std::piecewise_construct,
           std::forward_as_tuple(boost::forward<K>(k)),
           std::forward_as_tuple(boost::forward<Args>(args)...))

unlike emplace_hint, which simply forwards all arguments to value_type's constructor.

Can invalidate iterators, but only if the insert causes the load to be greater than the maximum load.

The template <class K, class…​ Args> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
insert_or_assign
template<class M>
  std::pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
template<class M>
  std::pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
template<class K, class M>
  std::pair<iterator, bool> insert_or_assign(K&& k, M&& obj);

Inserts a new element into the container or updates an existing one by assigning to the contained value.

If there is an element with key k, then it is updated by assigning boost::forward<M>(obj).

If there is no such element, it is added to the container as:

// first two overloads
value_type(std::piecewise_construct,
           std::forward_as_tuple(boost::forward<Key>(k)),
           std::forward_as_tuple(boost::forward<M>(obj)))

// third overload
value_type(std::piecewise_construct,
           std::forward_as_tuple(boost::forward<K>(k)),
           std::forward_as_tuple(boost::forward<M>(obj)))
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load to be greater than the maximum load.

The template<class K, class M> only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
insert_or_assign with Hint
template<class M>
  iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
template<class M>
  iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);
template<class K, class M>
  iterator insert_or_assign(const_iterator hint, K&& k, M&& obj);

Inserts a new element into the container or updates an existing one by assigning to the contained value.

If there is an element with key k, then it is updated by assigning boost::forward<M>(obj).

If there is no such element, it is added to the container as:

// first two overloads
value_type(std::piecewise_construct,
           std::forward_as_tuple(boost::forward<Key>(k)),
           std::forward_as_tuple(boost::forward<M>(obj)))

// third overload
value_type(std::piecewise_construct,
           std::forward_as_tuple(boost::forward<K>(k)),
           std::forward_as_tuple(boost::forward<M>(obj)))

hint is a suggestion to where the element should be inserted. This implementation ignores it.

Returns:

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load to be greater than the maximum load.

The template<class K, class M> only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
Erase by Position
void erase(iterator position);
void erase(const_iterator position);

Erase the element pointed to by position.

Throws:

Nothing.
Erase by Key
size_type erase(const key_type& k);
template<class K> size_type erase(K&& k);

Erase all elements with key equivalent to k.

Returns:

The number of elements erased.
Throws:

Only throws an exception if it is thrown by hasher or key_equal.
Notes:

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
Erase Range
iterator erase(const_iterator first, const_iterator last);

Erases the elements in the range from first to last.

Returns:

The iterator following the erased elements - i.e. last.
Throws:

Nothing in this implementation (neither the hasher nor the key_equal objects are called).
swap
void swap(unordered_node_map& other)
  noexcept(boost::allocator_traits<Allocator>::is_always_equal::value ||
           boost::allocator_traits<Allocator>::propagate_on_container_swap::value);

Swaps the contents of the container with the parameter.

If Allocator::propagate_on_container_swap is declared and Allocator::propagate_on_container_swap::value is true then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.

Throws:

Nothing unless key_equal or hasher throw on swapping.
Extract by Position
node_type extract(const_iterator position);

Extracts the element pointed to by position.

Returns:

A node_type object holding the extracted element.
Throws:

Nothing.
Extract by Key
node_type erase(const key_type& k);
template<class K> node_type erase(K&& k);

Extracts the element with key equivalent to k, if it exists.

Returns:

A node_type object holding the extracted element, or empty if no element was extracted.
Throws:

Only throws an exception if it is thrown by hasher or key_equal.
Notes:

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
clear
void clear() noexcept;

Erases all elements in the container.

Postconditions:

size() == 0, max_load() >= max_load_factor() * bucket_count()
merge
template<class H2, class P2>
  void merge(unordered_node_map<Key, T, H2, P2, Allocator>& source);
template<class H2, class P2>
  void merge(unordered_node_map<Key, T, H2, P2, Allocator>&& source);

Transfers all the element nodes from source whose key is not already present in *this.

Observers

get_allocator
allocator_type get_allocator() const noexcept;
Returns:

The container’s allocator.
hash_function
hasher hash_function() const;
Returns:

The container’s hash function.
key_eq
key_equal key_eq() const;
Returns:

The container’s key equality predicate

Lookup

find
iterator         find(const key_type& k);
const_iterator   find(const key_type& k) const;
template<class K>
  iterator       find(const K& k);
Returns:

An iterator pointing to an element with key equivalent to k, or end() if no such element exists.
Notes:

The template <typename K> overloads only participate in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
count
size_type        count(const key_type& k) const;
template<class K>
  size_type      count(const K& k) const;
Returns:

The number of elements with key equivalent to k.
Notes:

The template <typename K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
contains
bool             contains(const key_type& k) const;
template<class K>
  bool           contains(const K& k) const;
Returns:

A boolean indicating whether or not there is an element with key equal to key in the container
Notes:

The template <typename K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
equal_range
std::pair<iterator, iterator>               equal_range(const key_type& k);
std::pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
template<class K>
  std::pair<iterator, iterator>             equal_range(const K& k);
template<class K>
  std::pair<const_iterator, const_iterator> equal_range(const K& k) const;
Returns:

A range containing all elements with key equivalent to k. If the container doesn’t contain any such elements, returns std::make_pair(b.end(), b.end()).
Notes:

The template <typename K> overloads only participate in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
operator[]
mapped_type& operator[](const key_type& k);
mapped_type& operator[](key_type&& k);
template<class K> mapped_type& operator[](K&& k);
Effects:

If the container does not already contain an element with a key equivalent to k, inserts the value std::pair<key_type const, mapped_type>(k, mapped_type()).
Returns:

A reference to x.second where x is the element already in the container, or the newly inserted element with a key equivalent to k.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load to be greater than the maximum load.

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
at
mapped_type& at(const key_type& k);
const mapped_type& at(const key_type& k) const;
template<class K> mapped_type& at(const K& k);
template<class K> const mapped_type& at(const K& k) const;
Returns:

A reference to x.second where x is the (unique) element whose key is equivalent to k.
Throws:

An exception object of type std::out_of_range if no such element is present.
Notes:

The template<class K> overloads only participate in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.

Bucket Interface

bucket_count
size_type bucket_count() const noexcept;
Returns:

The size of the bucket array.

Hash Policy

load_factor
float load_factor() const noexcept;
Returns:

static_cast<float>(size())/static_cast<float>(bucket_count()), or 0 if bucket_count() == 0.
max_load_factor
float max_load_factor() const noexcept;
Returns:

Returns the container’s maximum load factor.
Set max_load_factor
void max_load_factor(float z);
Effects:

Does nothing, as the user is not allowed to change this parameter. Kept for compatibility with boost::unordered_map.
max_load
size_type max_load() const noexcept;
Returns:

The maximum number of elements the container can hold without rehashing, assuming that no further elements will be erased.
Note:

After construction, rehash or clearance, the container’s maximum load is at least max_load_factor() * bucket_count(). This number may decrease on erasure under high-load conditions.
rehash
void rehash(size_type n);

Changes if necessary the size of the bucket array so that there are at least n buckets, and so that the load factor is less than or equal to the maximum load factor. When applicable, this will either grow or shrink the bucket_count() associated with the container.

When size() == 0, rehash(0) will deallocate the underlying buckets array.

Invalidates iterators and changes the order of elements.

Throws:

The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function.
reserve
void reserve(size_type n);

Equivalent to a.rehash(ceil(n / a.max_load_factor())).

Similar to rehash, this function can be used to grow or shrink the number of buckets in the container.

Invalidates iterators and changes the order of elements.

Throws:

The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function.

Deduction Guides

A deduction guide will not participate in overload resolution if any of the following are true:

  • It has an InputIterator template parameter and a type that does not qualify as an input iterator is deduced for that parameter.

  • It has an Allocator template parameter and a type that does not qualify as an allocator is deduced for that parameter.

  • It has a Hash template parameter and an integral type or a type that qualifies as an allocator is deduced for that parameter.

  • It has a Pred template parameter and a type that qualifies as an allocator is deduced for that parameter.

A size_­type parameter type in a deduction guide refers to the size_­type member type of the container type deduced by the deduction guide. Its default value coincides with the default value of the constructor selected.

iter-value-type
template<class InputIterator>
  using iter-value-type =
    typename std::iterator_traits<InputIterator>::value_type; // exposition only
iter-key-type
template<class InputIterator>
  using iter-key-type = std::remove_const_t<
    std::tuple_element_t<0, iter-value-type<InputIterator>>>; // exposition only
iter-mapped-type
template<class InputIterator>
  using iter-mapped-type =
    std::tuple_element_t<1, iter-value-type<InputIterator>>;  // exposition only
iter-to-alloc-type
template<class InputIterator>
  using iter-to-alloc-type = std::pair<
    std::add_const_t<std::tuple_element_t<0, iter-value-type<InputIterator>>>,
    std::tuple_element_t<1, iter-value-type<InputIterator>>>; // exposition only

Equality Comparisons

operator==
template<class Key, class T, class Hash, class Pred, class Alloc>
  bool operator==(const unordered_node_map<Key, T, Hash, Pred, Alloc>& x,
                  const unordered_node_map<Key, T, Hash, Pred, Alloc>& y);

Return true if x.size() == y.size() and for every element in x, there is an element in y with the same key, with an equal value (using operator== to compare the value types).

Notes:

Behavior is undefined if the two containers don’t have equivalent equality predicates.
operator!=
template<class Key, class T, class Hash, class Pred, class Alloc>
  bool operator!=(const unordered_node_map<Key, T, Hash, Pred, Alloc>& x,
                  const unordered_node_map<Key, T, Hash, Pred, Alloc>& y);

Return false if x.size() == y.size() and for every element in x, there is an element in y with the same key, with an equal value (using operator== to compare the value types).

Notes:

Behavior is undefined if the two containers don’t have equivalent equality predicates.

Swap

template<class Key, class T, class Hash, class Pred, class Alloc>
  void swap(unordered_node_map<Key, T, Hash, Pred, Alloc>& x,
            unordered_node_map<Key, T, Hash, Pred, Alloc>& y)
    noexcept(noexcept(x.swap(y)));

Swaps the contents of x and y.

If Allocator::propagate_on_container_swap is declared and Allocator::propagate_on_container_swap::value is true then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.

Effects:

x.swap(y)
Throws:

Nothing unless key_equal or hasher throw on swapping.

erase_if

template<class K, class T, class H, class P, class A, class Predicate>
  typename unordered_node_map<K, T, H, P, A>::size_type
    erase_if(unordered_node_map<K, T, H, P, A>& c, Predicate pred);

Traverses the container c and removes all elements for which the supplied predicate returns true.

Returns:

The number of erased elements.
Notes:

Equivalent to:

auto original_size = c.size();
for (auto i = c.begin(), last = c.end(); i != last; ) {
  if (pred(*i)) {
    i = c.erase(i);
  } else {
    ++i;
  }
}
return original_size - c.size();

Class template unordered_node_set

boost::unordered_node_set — A node-based, open-addressing unordered associative container that stores unique values.

boost::unordered_node_set uses an open-addressing layout like boost::unordered_flat_set, but, being node-based, it provides pointer/iterator stability and node handling functionalities. Its performance lies between those of boost::unordered_set and boost::unordered_flat_set.

As a result of its using open addressing, the interface of boost::unordered_node_set deviates in a number of aspects from that of boost::unordered_set/std::unordered_set:

  • begin() is not constant-time.

  • erase(iterator) returns void.

  • There is no API for bucket handling (except bucket_count).

  • The maximum load factor of the container is managed internally and can’t be set by the user.

Other than this, boost::unordered_node_set is mostly a drop-in replacement of standard unordered associative containers.

Synopsis

// #include <boost/unordered/unordered_node_set.hpp>

namespace boost {
  template<class Key,
           class Hash = boost::hash<Key>,
           class Pred = std::equal_to<Key>,
           class Allocator = std::allocator<Key>>
  class unordered_node_set {
  public:
    // types
    using key_type             = Key;
    using value_type           = Key;
    using init_type            = Key;
    using hasher               = Hash;
    using key_equal            = Pred;
    using allocator_type       = Allocator;
    using pointer              = typename std::allocator_traits<Allocator>::pointer;
    using const_pointer        = typename std::allocator_traits<Allocator>::const_pointer;
    using reference            = value_type&;
    using const_reference      = const value_type&;
    using size_type            = std::size_t;
    using difference_type      = std::ptrdiff_t;

    using iterator             = implementation-defined;
    using const_iterator       = implementation-defined;

    using node_type            = implementation-defined;
    using insert_return_type   = implementation-defined;

    // construct/copy/destroy
    unordered_node_set();
    explicit unordered_node_set(size_type n,
                                const hasher& hf = hasher(),
                                const key_equal& eql = key_equal(),
                                const allocator_type& a = allocator_type());
    template<class InputIterator>
      unordered_node_set(InputIterator f, InputIterator l,
                         size_type n = implementation-defined,
                         const hasher& hf = hasher(),
                         const key_equal& eql = key_equal(),
                         const allocator_type& a = allocator_type());
    unordered_node_set(const unordered_node_set& other);
    unordered_node_set(unordered_node_set&& other);
    template<class InputIterator>
      unordered_node_set(InputIterator f, InputIterator l, const allocator_type& a);
    explicit unordered_node_set(const Allocator& a);
    unordered_node_set(const unordered_node_set& other, const Allocator& a);
    unordered_node_set(unordered_node_set&& other, const Allocator& a);
    unordered_node_set(std::initializer_list<value_type> il,
                       size_type n = implementation-defined
                       const hasher& hf = hasher(),
                       const key_equal& eql = key_equal(),
                       const allocator_type& a = allocator_type());
    unordered_node_set(size_type n, const allocator_type& a);
    unordered_node_set(size_type n, const hasher& hf, const allocator_type& a);
    template<class InputIterator>
      unordered_node_set(InputIterator f, InputIterator l, size_type n, const allocator_type& a);
    template<class InputIterator>
      unordered_node_set(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                         const allocator_type& a);
    unordered_node_set(std::initializer_list<value_type> il, const allocator_type& a);
    unordered_node_set(std::initializer_list<value_type> il, size_type n,
                       const allocator_type& a);
    unordered_node_set(std::initializer_list<value_type> il, size_type n, const hasher& hf,
                       const allocator_type& a);
    ~unordered_node_set();
    unordered_node_set& operator=(const unordered_node_set& other);
    unordered_node_set& operator=(unordered_node_set&& other)
      noexcept(boost::allocator_traits<Allocator>::is_always_equal::value ||
              boost::allocator_traits<Allocator>::propagate_on_container_move_assignment::value);
    unordered_node_set& operator=(std::initializer_list<value_type>);
    allocator_type get_allocator() const noexcept;

    // iterators
    iterator       begin() noexcept;
    const_iterator begin() const noexcept;
    iterator       end() noexcept;
    const_iterator end() const noexcept;
    const_iterator cbegin() const noexcept;
    const_iterator cend() const noexcept;

    // capacity
    [[nodiscard]] bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;

    // modifiers
    template<class... Args> std::pair<iterator, bool> emplace(Args&&... args);
    template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
    std::pair<iterator, bool> insert(const value_type& obj);
    std::pair<iterator, bool> insert(value_type&& obj);
    template<class K> std::pair<iterator, bool> insert(K&& k);
    iterator insert(const_iterator hint, const value_type& obj);
    iterator insert(const_iterator hint, value_type&& obj);
    template<class K> iterator insert(const_iterator hint, K&& k);
    template<class InputIterator> void insert(InputIterator first, InputIterator last);
    void insert(std::initializer_list<value_type>);
    insert_return_type insert(node_type&& nh);
    iterator insert(const_iterator hint, node_type&& nh);

    void      erase(iterator position);
    void      erase(const_iterator position);
    size_type erase(const key_type& k);
    template<class K> size_type erase(K&& k);
    iterator  erase(const_iterator first, const_iterator last);
    void      swap(unordered_node_set& other)
      noexcept(boost::allocator_traits<Allocator>::is_always_equal::value ||
               boost::allocator_traits<Allocator>::propagate_on_container_swap::value);
    node_type extract(const_iterator position);
    node_type extract(const key_type& key);
    template<class K> node_type extract(K&& key);
    void      clear() noexcept;

    template<class H2, class P2>
      void merge(unordered_node_set<Key, T, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_node_set<Key, T, H2, P2, Allocator>&& source);

    // observers
    hasher hash_function() const;
    key_equal key_eq() const;

    // set operations
    iterator         find(const key_type& k);
    const_iterator   find(const key_type& k) const;
    template<class K>
      iterator       find(const K& k);
    template<class K>
      const_iterator find(const K& k) const;
    size_type        count(const key_type& k) const;
    template<class K>
      size_type      count(const K& k) const;
    bool             contains(const key_type& k) const;
    template<class K>
      bool           contains(const K& k) const;
    std::pair<iterator, iterator>               equal_range(const key_type& k);
    std::pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
    template<class K>
      std::pair<iterator, iterator>             equal_range(const K& k);
    template<class K>
      std::pair<const_iterator, const_iterator> equal_range(const K& k) const;

    // bucket interface
    size_type bucket_count() const noexcept;

    // hash policy
    float load_factor() const noexcept;
    float max_load_factor() const noexcept;
    void max_load_factor(float z);
    size_type max_load() const noexcept;
    void rehash(size_type n);
    void reserve(size_type n);
  };

  // Deduction Guides
  template<class InputIterator,
           class Hash = boost::hash<iter-value-type<InputIterator>>,
           class Pred = std::equal_to<iter-value-type<InputIterator>>,
           class Allocator = std::allocator<iter-value-type<InputIterator>>>
    unordered_node_set(InputIterator, InputIterator, typename see below::size_type = see below,
                       Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_node_set<iter-value-type<InputIterator>, Hash, Pred, Allocator>;

  template<class T, class Hash = boost::hash<T>, class Pred = std::equal_to<T>,
           class Allocator = std::allocator<T>>
    unordered_node_set(std::initializer_list<T>, typename see below::size_type = see below,
                       Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_node_set<T, Hash, Pred, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_node_set(InputIterator, InputIterator, typename see below::size_type, Allocator)
      -> unordered_node_set<iter-value-type<InputIterator>,
                            boost::hash<iter-value-type<InputIterator>>,
                            std::equal_to<iter-value-type<InputIterator>>, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_node_set(InputIterator, InputIterator, Allocator)
      -> unordered_node_set<iter-value-type<InputIterator>,
                            boost::hash<iter-value-type<InputIterator>>,
                            std::equal_to<iter-value-type<InputIterator>>, Allocator>;

  template<class InputIterator, class Hash, class Allocator>
    unordered_node_set(InputIterator, InputIterator, typename see below::size_type, Hash,
                       Allocator)
      -> unordered_node_set<iter-value-type<InputIterator>, Hash,
                            std::equal_to<iter-value-type<InputIterator>>, Allocator>;

  template<class T, class Allocator>
    unordered_node_set(std::initializer_list<T>, typename see below::size_type, Allocator)
      -> unordered_node_set<T, boost::hash<T>, std::equal_to<T>, Allocator>;

  template<class T, class Allocator>
    unordered_node_set(std::initializer_list<T>, Allocator)
      -> unordered_node_set<T, boost::hash<T>, std::equal_to<T>, Allocator>;

  template<class T, class Hash, class Allocator>
    unordered_node_set(std::initializer_list<T>, typename see below::size_type, Hash, Allocator)
      -> unordered_node_set<T, Hash, std::equal_to<T>, Allocator>;

  // Equality Comparisons
  template<class Key, class T, class Hash, class Pred, class Alloc>
    bool operator!=(const unordered_node_set<Key, T, Hash, Pred, Alloc>& x,
                    const unordered_node_set<Key, T, Hash, Pred, Alloc>& y);

  template<class Key, class T, class Hash, class Pred, class Alloc>
    bool operator!=(const unordered_node_set<Key, T, Hash, Pred, Alloc>& x,
                    const unordered_node_set<Key, T, Hash, Pred, Alloc>& y);

  // swap
  template<class Key, class T, class Hash, class Pred, class Alloc>
    void swap(unordered_node_set<Key, T, Hash, Pred, Alloc>& x,
              unordered_node_set<Key, T, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));

  template<class K, class T, class H, class P, class A, class Predicate>
    typename unordered_node_set<K, T, H, P, A>::size_type
       erase_if(unordered_node_set<K, T, H, P, A>& c, Predicate pred);
}

Description

Template Parameters

Key

Key must be Erasable from the container.

Hash

A unary function object type that acts a hash function for a Key. It takes a single argument of type Key and returns a value of type std::size_t.

Pred

A binary function object that induces an equivalence relation on values of type Key. It takes two arguments of type Key and returns a value of type bool.

Allocator

An allocator whose value type is the same as the container’s value type. std::allocator_traits<Allocator>::pointer and std::allocator_traits<Allocator>::const_pointer must be convertible to/from value_type* and const value_type*, respectively.

The element nodes of the container are held into an internal bucket array. A node is inserted into a bucket determined by the hash code of its element, but if the bucket is already occupied (a collision), an available one in the vicinity of the original position is used.

The size of the bucket array can be automatically increased by a call to insert/emplace, or as a result of calling rehash/reserve. The load factor of the container (number of elements divided by number of buckets) is never greater than max_load_factor(), except possibly for small sizes where the implementation may decide to allow for higher loads.

If hash_is_avalanching<Hash>::value is true, the hash function is used as-is; otherwise, a bit-mixing post-processing stage is added to increase the quality of hashing at the expense of extra computational cost.

Typedefs

typedef implementation-defined iterator;

A constant iterator whose value type is value_type.

The iterator category is at least a forward iterator.

Convertible to const_iterator.

typedef implementation-defined const_iterator;

A constant iterator whose value type is value_type.

The iterator category is at least a forward iterator.

typedef implementation-defined node_type;

A class for holding extracted container elements, modelling NodeHandle.

typedef implementation-defined insert_return_type;

A specialization of an internal class template:

template<class Iterator, class NodeType>
struct insert_return_type // name is exposition only
{
  Iterator position;
  bool     inserted;
  NodeType node;
};

with Iterator = iterator and NodeType = node_type.

Constructors

Default Constructor
unordered_node_set();

Constructs an empty container using hasher() as the hash function, key_equal() as the key equality predicate and allocator_type() as the allocator.

Postconditions:

size() == 0
Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Bucket Count Constructor
explicit unordered_node_set(size_type n,
                            const hasher& hf = hasher(),
                            const key_equal& eql = key_equal(),
                            const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate, and a as the allocator.

Postconditions:

size() == 0
Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Iterator Range Constructor
template<class InputIterator>
  unordered_node_set(InputIterator f, InputIterator l,
                     size_type n = implementation-defined,
                     const hasher& hf = hasher(),
                     const key_equal& eql = key_equal(),
                     const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate and a as the allocator, and inserts the elements from [f, l) into it.

Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Copy Constructor
unordered_node_set(unordered_node_set const& other);

The copy constructor. Copies the contained elements, hash function, predicate and allocator.

If Allocator::select_on_container_copy_construction exists and has the right signature, the allocator will be constructed from its result.

Requires:

value_type is copy constructible
Move Constructor
unordered_node_set(unordered_node_set&& other);

The move constructor. The internal bucket array of other is transferred directly to the new container. The hash function, predicate and allocator are moved-constructed from other.

Iterator Range Constructor with Allocator
template<class InputIterator>
  unordered_node_set(InputIterator f, InputIterator l, const allocator_type& a);

Constructs an empty container using a as the allocator, with the default hash function and key equality predicate and inserts the elements from [f, l) into it.

Requires:

hasher, key_equal need to be DefaultConstructible.
Allocator Constructor
explicit unordered_node_set(Allocator const& a);

Constructs an empty container, using allocator a.

Copy Constructor with Allocator
unordered_node_set(unordered_node_set const& other, Allocator const& a);

Constructs a container, copying other's contained elements, hash function, and predicate, but using allocator a.

Move Constructor with Allocator
unordered_node_set(unordered_node_set&& other, Allocator const& a);

If a == other.get_allocator(), the element nodes of other are transferred directly to the new container; otherwise, elements are moved-constructed from those of other. The hash function and predicate are moved-constructed from other, and the allocator is copy-constructed from a.

Initializer List Constructor
unordered_node_set(std::initializer_list<value_type> il,
              size_type n = implementation-defined
              const hasher& hf = hasher(),
              const key_equal& eql = key_equal(),
              const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate and a, and inserts the elements from il into it.

Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.
Bucket Count Constructor with Allocator
unordered_node_set(size_type n, allocator_type const& a);

Constructs an empty container with at least n buckets, using hf as the hash function, the default hash function and key equality predicate and a as the allocator.

Postconditions:

size() == 0
Requires:

hasher and key_equal need to be DefaultConstructible.
Bucket Count Constructor with Hasher and Allocator
unordered_node_set(size_type n, hasher const& hf, allocator_type const& a);

Constructs an empty container with at least n buckets, using hf as the hash function, the default key equality predicate and a as the allocator.

Postconditions:

size() == 0
Requires:

key_equal needs to be DefaultConstructible.
Iterator Range Constructor with Bucket Count and Allocator
template<class InputIterator>
  unordered_node_set(InputIterator f, InputIterator l, size_type n, const allocator_type& a);

Constructs an empty container with at least n buckets, using a as the allocator and default hash function and key equality predicate, and inserts the elements from [f, l) into it.

Requires:

hasher, key_equal need to be DefaultConstructible.
Iterator Range Constructor with Bucket Count and Hasher
    template<class InputIterator>
      unordered_node_set(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                         const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, a as the allocator, with the default key equality predicate, and inserts the elements from [f, l) into it.

Requires:

key_equal needs to be DefaultConstructible.
initializer_list Constructor with Allocator
unordered_node_set(std::initializer_list<value_type> il, const allocator_type& a);

Constructs an empty container using a and default hash function and key equality predicate, and inserts the elements from il into it.

Requires:

hasher and key_equal need to be DefaultConstructible.
initializer_list Constructor with Bucket Count and Allocator
unordered_node_set(std::initializer_list<value_type> il, size_type n, const allocator_type& a);

Constructs an empty container with at least n buckets, using a and default hash function and key equality predicate, and inserts the elements from il into it.

Requires:

hasher and key_equal need to be DefaultConstructible.
initializer_list Constructor with Bucket Count and Hasher and Allocator
unordered_node_set(std::initializer_list<value_type> il, size_type n, const hasher& hf,
                   const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, a as the allocator and default key equality predicate,and inserts the elements from il into it.

Requires:

key_equal needs to be DefaultConstructible.

Destructor

~unordered_node_set();
Note:

The destructor is applied to every element, and all memory is deallocated

Assignment

Copy Assignment
unordered_node_set& operator=(unordered_node_set const& other);

The assignment operator. Destroys previously existing elements, copy-assigns the hash function and predicate from other, copy-assigns the allocator from other if Alloc::propagate_on_container_copy_assignment exists and Alloc::propagate_on_container_copy_assignment::value is true, and finally inserts copies of the elements of other.

Requires:

value_type is CopyInsertable
Move Assignment
unordered_node_set& operator=(unordered_node_set&& other)
  noexcept(boost::allocator_traits<Allocator>::is_always_equal::value ||
           boost::allocator_traits<Allocator>::propagate_on_container_move_assignment::value);

The move assignment operator. Destroys previously existing elements, swaps the hash function and predicate from other, and move-assigns the allocator from other if Alloc::propagate_on_container_move_assignment exists and Alloc::propagate_on_container_move_assignment::value is true. If at this point the allocator is equal to other.get_allocator(), the internal bucket array of other is transferred directly to the new container; otherwise, inserts move-constructed copies of the elements of other.

Initializer List Assignment
unordered_node_set& operator=(std::initializer_list<value_type> il);

Assign from values in initializer list. All previously existing elements are destroyed.

Requires:

value_type is CopyInsertable

Iterators

begin
iterator begin() noexcept;
const_iterator begin() const noexcept;
Returns:

An iterator referring to the first element of the container, or if the container is empty the past-the-end value for the container.
Complexity:

O(bucket_count())
end
iterator end() noexcept;
const_iterator end() const noexcept;
Returns:

An iterator which refers to the past-the-end value for the container.
cbegin
const_iterator cbegin() const noexcept;
Returns:

A const_iterator referring to the first element of the container, or if the container is empty the past-the-end value for the container.
Complexity:

O(bucket_count())
cend
const_iterator cend() const noexcept;
Returns:

A const_iterator which refers to the past-the-end value for the container.

Size and Capacity

empty
[[nodiscard]] bool empty() const noexcept;
Returns:

size() == 0
size
size_type size() const noexcept;
Returns:

std::distance(begin(), end())
max_size
size_type max_size() const noexcept;
Returns:

size() of the largest possible container.

Modifiers

emplace
template<class... Args> std::pair<iterator, bool> emplace(Args&&... args);

Inserts an object, constructed with the arguments args, in the container if and only if there is no element in the container with an equivalent key.

Requires:

value_type is constructible from args.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load to be greater than the maximum load.
emplace_hint
    template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);

Inserts an object, constructed with the arguments args, in the container if and only if there is no element in the container with an equivalent key.

position is a suggestion to where the element should be inserted. This implementation ignores it.

Requires:

value_type is constructible from args.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load to be greater than the maximum load.
Copy Insert
std::pair<iterator, bool> insert(const value_type& obj);

Inserts obj in the container if and only if there is no element in the container with an equivalent key.

Requires:

value_type is CopyInsertable.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load to be greater than the maximum load.
Move Insert
std::pair<iterator, bool> insert(value_type&& obj);

Inserts obj in the container if and only if there is no element in the container with an equivalent key.

Requires:

value_type is MoveInsertable.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load to be greater than the maximum load.
Transparent Insert
template<class K> std::pair<iterator, bool> insert(K&& k);

Inserts an element constructed from std::forward<K>(k) in the container if and only if there is no element in the container with an equivalent key.

Requires:

value_type is EmplaceConstructible from k.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load to be greater than the maximum load.

This overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
Copy Insert with Hint
iterator insert(const_iterator hint, const value_type& obj);

Inserts obj in the container if and only if there is no element in the container with an equivalent key.

hint is a suggestion to where the element should be inserted. This implementation ignores it.

Requires:

value_type is CopyInsertable.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load to be greater than the maximum load.
Move Insert with Hint
iterator insert(const_iterator hint, value_type&& obj);

Inserts obj in the container if and only if there is no element in the container with an equivalent key.

hint is a suggestion to where the element should be inserted. This implementation ignores it.

Requires:

value_type is MoveInsertable.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load to be greater than the maximum load.
Transparent Insert with Hint
template<class K> std::pair<iterator, bool> insert(const_iterator hint, K&& k);

Inserts an element constructed from std::forward<K>(k) in the container if and only if there is no element in the container with an equivalent key.

hint is a suggestion to where the element should be inserted. This implementation ignores it.

Requires:

value_type is EmplaceConstructible from k.
Returns:

The bool component of the return type is true if an insert took place.

If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load to be greater than the maximum load.

This overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
Insert Iterator Range
template<class InputIterator> void insert(InputIterator first, InputIterator last);

Inserts a range of elements into the container. Elements are inserted if and only if there is no element in the container with an equivalent key.

Requires:

value_type is EmplaceConstructible into the container from *first.
Throws:

When inserting a single element, if an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load to be greater than the maximum load.
Insert Initializer List
void insert(std::initializer_list<value_type>);

Inserts a range of elements into the container. Elements are inserted if and only if there is no element in the container with an equivalent key.

Requires:

value_type is EmplaceConstructible into the container from *first.
Throws:

When inserting a single element, if an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Can invalidate iterators, but only if the insert causes the load to be greater than the maximum load.
Insert Node
insert_return_type insert(node_type&& nh);

If nh is not empty, inserts the associated element in the container if and only if there is no element in the container with a key equivalent to nh.value(). nh is empty when the function returns.

Returns:

An insert_return_type object constructed from position, inserted and node:

  • If nh is empty, inserted is false, position is end(), and node is empty.

  • Otherwise if the insertion took place, inserted is true, position points to the inserted element, and node is empty.

  • If the insertion failed, inserted is false, node has the previous value of nh, and position points to an element with a key equivalent to nh.value().
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Behavior is undefined if nh is not empty and the allocators of nh and the container are not equal.
Insert Node with Hint
iterator insert(const_iterator hint, node_type&& nh);

If nh is not empty, inserts the associated element in the container if and only if there is no element in the container with a key equivalent to nh.value(). nh becomes empty if insertion took place, otherwise it is not changed.

hint is a suggestion to where the element should be inserted. This implementation ignores it.

Returns:

The iterator returned is end() if nh is empty. If insertion took place, then the iterator points to the newly inserted element; otherwise, it points to the element with equivalent key.
Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.
Notes:

Behavior is undefined if nh is not empty and the allocators of nh and the container are not equal.
Erase by Position
void erase(iterator position);
void erase(const_iterator position);

Erase the element pointed to by position.

Throws:

Nothing.
Erase by Key
size_type erase(const key_type& k);
template<class K> size_type erase(K&& k);

Erase all elements with key equivalent to k.

Returns:

The number of elements erased.
Throws:

Only throws an exception if it is thrown by hasher or key_equal.
Notes:

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
Erase Range
iterator erase(const_iterator first, const_iterator last);

Erases the elements in the range from first to last.

Returns:

The iterator following the erased elements - i.e. last.
Throws:

Nothing in this implementation (neither the hasher nor the key_equal objects are called).
swap
void swap(unordered_node_set& other)
  noexcept(boost::allocator_traits<Allocator>::is_always_equal::value ||
           boost::allocator_traits<Allocator>::propagate_on_container_swap::value);

Swaps the contents of the container with the parameter.

If Allocator::propagate_on_container_swap is declared and Allocator::propagate_on_container_swap::value is true then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.

Throws:

Nothing unless key_equal or hasher throw on swapping.
Extract by Position
node_type extract(const_iterator position);

Extracts the element pointed to by position.

Returns:

A node_type object holding the extracted element.
Throws:

Nothing.
Extract by Key
node_type erase(const key_type& k);
template<class K> node_type erase(K&& k);

Extracts the element with key equivalent to k, if it exists.

Returns:

A node_type object holding the extracted element, or empty if no element was extracted.
Throws:

Only throws an exception if it is thrown by hasher or key_equal.
Notes:

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
clear
void clear() noexcept;

Erases all elements in the container.

Postconditions:

size() == 0, max_load() >= max_load_factor() * bucket_count()
merge
template<class H2, class P2>
  void merge(unordered_node_set<Key, T, H2, P2, Allocator>& source);
template<class H2, class P2>
  void merge(unordered_node_set<Key, T, H2, P2, Allocator>&& source);

Transfers all the element nodes from source whose key is not already present in *this.

Observers

get_allocator
allocator_type get_allocator() const noexcept;
Returns:

The container’s allocator.
hash_function
hasher hash_function() const;
Returns:

The container’s hash function.
key_eq
key_equal key_eq() const;
Returns:

The container’s key equality predicate

Lookup

find
iterator         find(const key_type& k);
const_iterator   find(const key_type& k) const;
template<class K>
  iterator       find(const K& k);
Returns:

An iterator pointing to an element with key equivalent to k, or end() if no such element exists.
Notes:

The template <typename K> overloads only participate in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
count
size_type        count(const key_type& k) const;
template<class K>
  size_type      count(const K& k) const;
Returns:

The number of elements with key equivalent to k.
Notes:

The template <typename K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
contains
bool             contains(const key_type& k) const;
template<class K>
  bool           contains(const K& k) const;
Returns:

A boolean indicating whether or not there is an element with key equal to key in the container
Notes:

The template <typename K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.
equal_range
std::pair<iterator, iterator>               equal_range(const key_type& k);
std::pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
template<class K>
  std::pair<iterator, iterator>             equal_range(const K& k);
template<class K>
  std::pair<const_iterator, const_iterator> equal_range(const K& k) const;
Returns:

A range containing all elements with key equivalent to k. If the container doesn’t contain any such elements, returns std::make_pair(b.end(), b.end()).
Notes:

The template <typename K> overloads only participate in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.

Bucket Interface

bucket_count
size_type bucket_count() const noexcept;
Returns:

The size of the bucket array.

Hash Policy

load_factor
float load_factor() const noexcept;
Returns:

static_cast<float>(size())/static_cast<float>(bucket_count()), or 0 if bucket_count() == 0.
max_load_factor
float max_load_factor() const noexcept;
Returns:

Returns the container’s maximum load factor.
Set max_load_factor
void max_load_factor(float z);
Effects:

Does nothing, as the user is not allowed to change this parameter. Kept for compatibility with boost::unordered_set.
max_load
size_type max_load() const noexcept;
Returns:

The maximum number of elements the container can hold without rehashing, assuming that no further elements will be erased.
Note:

After construction, rehash or clearance, the container’s maximum load is at least max_load_factor() * bucket_count(). This number may decrease on erasure under high-load conditions.
rehash
void rehash(size_type n);

Changes if necessary the size of the bucket array so that there are at least n buckets, and so that the load factor is less than or equal to the maximum load factor. When applicable, this will either grow or shrink the bucket_count() associated with the container.

When size() == 0, rehash(0) will deallocate the underlying buckets array.

Invalidates iterators and changes the order of elements.

Throws:

The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function.
reserve
void reserve(size_type n);

Equivalent to a.rehash(ceil(n / a.max_load_factor())).

Similar to rehash, this function can be used to grow or shrink the number of buckets in the container.

Invalidates iterators and changes the order of elements.

Throws:

The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function.

Deduction Guides

A deduction guide will not participate in overload resolution if any of the following are true:

  • It has an InputIterator template parameter and a type that does not qualify as an input iterator is deduced for that parameter.

  • It has an Allocator template parameter and a type that does not qualify as an allocator is deduced for that parameter.

  • It has a Hash template parameter and an integral type or a type that qualifies as an allocator is deduced for that parameter.

  • It has a Pred template parameter and a type that qualifies as an allocator is deduced for that parameter.

A size_­type parameter type in a deduction guide refers to the size_­type member type of the container type deduced by the deduction guide. Its default value coincides with the default value of the constructor selected.

iter-value-type
template<class InputIterator>
  using iter-value-type =
    typename std::iterator_traits<InputIterator>::value_type; // exposition only

Equality Comparisons

operator==
template<class Key, class T, class Hash, class Pred, class Alloc>
  bool operator==(const unordered_node_set<Key, T, Hash, Pred, Alloc>& x,
                  const unordered_node_set<Key, T, Hash, Pred, Alloc>& y);

Return true if x.size() == y.size() and for every element in x, there is an element in y with the same key, with an equal value (using operator== to compare the value types).

Notes:

Behavior is undefined if the two containers don’t have equivalent equality predicates.
operator!=
template<class Key, class T, class Hash, class Pred, class Alloc>
  bool operator!=(const unordered_node_set<Key, T, Hash, Pred, Alloc>& x,
                  const unordered_node_set<Key, T, Hash, Pred, Alloc>& y);

Return false if x.size() == y.size() and for every element in x, there is an element in y with the same key, with an equal value (using operator== to compare the value types).

Notes:

Behavior is undefined if the two containers don’t have equivalent equality predicates.

Swap

template<class Key, class T, class Hash, class Pred, class Alloc>
  void swap(unordered_node_set<Key, T, Hash, Pred, Alloc>& x,
            unordered_node_set<Key, T, Hash, Pred, Alloc>& y)
    noexcept(noexcept(x.swap(y)));

Swaps the contents of x and y.

If Allocator::propagate_on_container_swap is declared and Allocator::propagate_on_container_swap::value is true then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.

Effects:

x.swap(y)
Throws:

Nothing unless key_equal or hasher throw on swapping.

erase_if

template<class K, class T, class H, class P, class A, class Predicate>
  typename unordered_node_set<K, T, H, P, A>::size_type
    erase_if(unordered_node_set<K, T, H, P, A>& c, Predicate pred);

Traverses the container c and removes all elements for which the supplied predicate returns true.

Returns:

The number of erased elements.
Notes:

Equivalent to:

auto original_size = c.size();
for (auto i = c.begin(), last = c.end(); i != last; ) {
  if (pred(*i)) {
    i = c.erase(i);
  } else {
    ++i;
  }
}
return original_size - c.size();

Change Log

Release 1.82.0 - Major update

  • C++03 support is planned for deprecation. Boost 1.84.0 will no longer support C++03 mode and C++11 will become the new minimum for using the library.

  • Added node-based, open-addressing containers boost::unordered_node_map and boost::unordered_node_set.

  • Extended heterogeneous lookup to more member functions as specified in P2363.

  • Replaced the previous post-mixing process for open-addressing containers with a new algorithm based on extended multiplication by a constant.

  • Fixed bug in internal emplace() impl where stack-local types were not properly constructed using the Allocator of the container which breaks uses-allocator construction.

Release 1.81.0 - Major update

  • Added fast containers boost::unordered_flat_map and boost::unordered_flat_set based on open addressing.

  • Added CTAD deduction guides for all containers.

  • Added missing constructors as specified in LWG issue 2713.

Release 1.80.0 - Major update

  • Refactor internal implementation to be dramatically faster

  • Allow final Hasher and KeyEqual objects

  • Update documentation, adding benchmark graphs and notes on the new internal data structures

Release 1.79.0

  • Improved C++20 support:

    • All containers have been updated to support heterogeneous count, equal_range and find.

    • All containers now implement the member function contains.

    • erase_if has been implemented for all containers.

  • Improved C++23 support:

    • All containers have been updated to support heterogeneous erase and extract.

  • Changed behavior of reserve to eagerly allocate (PR#59).

  • Various warning fixes in the test suite.

  • Update code to internally use boost::allocator_traits.

  • Switch to Fibonacci hashing.

  • Update documentation to be written in AsciiDoc instead of QuickBook.

Release 1.67.0

  • Improved C++17 support:

    • Add template deduction guides from the standard.

    • Use a simple implementation of optional in node handles, so that they’re closer to the standard.

    • Add missing noexcept specifications to swap, operator= and node handles, and change the implementation to match. Using std::allocator_traits::is_always_equal, or our own implementation when not available, and boost::is_nothrow_swappable in the implementation.

  • Improved C++20 support:

    • Use boost::to_address, which has the proposed C++20 semantics, rather than the old custom implementation.

  • Add element_type to iterators, so that std::pointer_traits will work.

  • Use std::piecewise_construct on recent versions of Visual C++, and other uses of the Dinkumware standard library, now using Boost.Predef to check compiler and library versions.

  • Use std::iterator_traits rather than the boost iterator traits in order to remove dependency on Boost.Iterator.

  • Remove iterators' inheritance from std::iterator, which is deprecated in C++17, thanks to Daniela Engert (PR#7).

  • Stop using BOOST_DEDUCED_TYPENAME.

  • Update some Boost include paths.

  • Rename some internal methods, and variables.

  • Various testing improvements.

  • Miscellaneous internal changes.

Release 1.66.0

  • Simpler move construction implementation.

  • Documentation fixes (GitHub #6).

Release 1.65.0

  • Add deprecated attributes to quick_erase and erase_return_void. I really will remove them in a future version this time.

  • Small standards compliance fixes:

    • noexpect specs for swap free functions.

    • Add missing insert(P&&) methods.

Release 1.64.0

  • Initial support for new C++17 member functions: insert_or_assign and try_emplace in unordered_map,

  • Initial support for merge and extract. Does not include transferring nodes between unordered_map and unordered_multimap or between unordered_set and unordered_multiset yet. That will hopefully be in the next version of Boost.

Release 1.63.0

  • Check hint iterator in insert/emplace_hint.

  • Fix some warnings, mostly in the tests.

  • Manually write out emplace_args for small numbers of arguments - should make template error messages a little more bearable.

  • Remove superfluous use of boost::forward in emplace arguments, which fixes emplacing string literals in old versions of Visual C++.

  • Fix an exception safety issue in assignment. If bucket allocation throws an exception, it can overwrite the hash and equality functions while leaving the existing elements in place. This would mean that the function objects wouldn’t match the container elements, so elements might be in the wrong bucket and equivalent elements would be incorrectly handled.

  • Various reference documentation improvements.

  • Better allocator support (#12459).

  • Make the no argument constructors implicit.

  • Implement missing allocator aware constructors.

  • Fix assigning the hash/key equality functions for empty containers.

  • Remove unary/binary_function from the examples in the documentation. They are removed in C++17.

  • Support 10 constructor arguments in emplace. It was meant to support up to 10 arguments, but an off by one error in the preprocessor code meant it only supported up to 9.

Release 1.62.0

  • Remove use of deprecated boost::iterator.

  • Remove BOOST_NO_STD_DISTANCE workaround.

  • Remove BOOST_UNORDERED_DEPRECATED_EQUALITY warning.

  • Simpler implementation of assignment, fixes an exception safety issue for unordered_multiset and unordered_multimap. Might be a little slower.

  • Stop using return value SFINAE which some older compilers have issues with.

Release 1.58.0

  • Remove unnecessary template parameter from const iterators.

  • Rename private iterator typedef in some iterator classes, as it confuses some traits classes.

  • Fix move assignment with stateful, propagate_on_container_move_assign allocators (#10777).

  • Fix rare exception safety issue in move assignment.

  • Fix potential overflow when calculating number of buckets to allocate (GitHub #4).

Release 1.57.0

  • Fix the pointer typedef in iterators (#10672).

  • Fix Coverity warning (GitHub #2).

Release 1.56.0

  • Fix some shadowed variable warnings (#9377).

  • Fix allocator use in documentation (#9719).

  • Always use prime number of buckets for integers. Fixes performance regression when inserting consecutive integers, although makes other uses slower (#9282).

  • Only construct elements using allocators, as specified in C++11 standard.

Release 1.55.0

  • Avoid some warnings (#8851, #8874).

  • Avoid exposing some detail functions via. ADL on the iterators.

  • Follow the standard by only using the allocators' construct and destroy methods to construct and destroy stored elements. Don’t use them for internal data like pointers.

Release 1.54.0

  • Mark methods specified in standard as noexpect. More to come in the next release.

  • If the hash function and equality predicate are known to both have nothrow move assignment or construction then use them.

Release 1.53.0

  • Remove support for the old pre-standard variadic pair constructors, and equality implementation. Both have been deprecated since Boost 1.48.

  • Remove use of deprecated config macros.

  • More internal implementation changes, including a much simpler implementation of erase.

Release 1.52.0

  • Faster assign, which assigns to existing nodes where possible, rather than creating entirely new nodes and copy constructing.

  • Fixed bug in erase_range (#7471).

  • Reverted some of the internal changes to how nodes are created, especially for C++11 compilers. 'construct' and 'destroy' should work a little better for C++11 allocators.

  • Simplified the implementation a bit. Hopefully more robust.

Release 1.51.0

  • Fix construction/destruction issue when using a C++11 compiler with a C++03 allocator (#7100).

  • Remove a try..catch to support compiling without exceptions.

  • Adjust SFINAE use to try to support g++ 3.4 (#7175).

  • Updated to use the new config macros.

Release 1.50.0

  • Fix equality for unordered_multiset and unordered_multimap.

  • Ticket 6857: Implement reserve.

  • Ticket 6771: Avoid gcc’s -Wfloat-equal warning.

  • Ticket 6784: Fix some Sun specific code.

  • Ticket 6190: Avoid gcc’s -Wshadow warning.

  • Ticket 6905: Make namespaces in macros compatible with bcp custom namespaces. Fixed by Luke Elliott.

  • Remove some of the smaller prime number of buckets, as they may make collisions quite probable (e.g. multiples of 5 are very common because we used base 10).

  • On old versions of Visual C++, use the container library’s implementation of allocator_traits, as it’s more likely to work.

  • On machines with 64 bit std::size_t, use power of 2 buckets, with Thomas Wang’s hash function to pick which one to use. As modulus is very slow for 64 bit values.

  • Some internal changes.

Release 1.49.0

  • Fix warning due to accidental odd assignment.

  • Slightly better error messages.

Release 1.48.0 - Major update

This is major change which has been converted to use Boost.Move’s move emulation, and be more compliant with the C++11 standard. See the compliance section for details.

The container now meets C++11’s complexity requirements, but to do so uses a little more memory. This means that quick_erase and erase_return_void are no longer required, they’ll be removed in a future version.

C++11 support has resulted in some breaking changes:

  • Equality comparison has been changed to the C++11 specification. In a container with equivalent keys, elements in a group with equal keys used to have to be in the same order to be considered equal, now they can be a permutation of each other. To use the old behavior define the macro BOOST_UNORDERED_DEPRECATED_EQUALITY.

  • The behaviour of swap is different when the two containers to be swapped has unequal allocators. It used to allocate new nodes using the appropriate allocators, it now swaps the allocators if the allocator has a member structure propagate_on_container_swap, such that propagate_on_container_swap::value is true.

  • Allocator’s construct and destroy functions are called with raw pointers, rather than the allocator’s pointer type.

  • emplace used to emulate the variadic pair constructors that appeared in early C++0x drafts. Since they were removed it no longer does so. It does emulate the new piecewise_construct pair constructors - only you need to use boost::piecewise_construct. To use the old emulation of the variadic constructors define BOOST_UNORDERED_DEPRECATED_PAIR_CONSTRUCT.

Release 1.45.0

  • Fix a bug when inserting into an unordered_map or unordered_set using iterators which returns value_type by copy.

Release 1.43.0

  • Ticket 3966: erase_return_void is now quick_erase, which is the current forerunner for resolving the slow erase by iterator, although there’s a strong possibility that this may change in the future. The old method name remains for backwards compatibility but is considered deprecated and will be removed in a future release.

  • Use Boost.Exception.

  • Stop using deprecated BOOST_HAS_* macros.

Release 1.42.0

  • Support instantiating the containers with incomplete value types.

  • Reduced the number of warnings (mostly in tests).

  • Improved codegear compatibility.

  • Ticket 3693: Add erase_return_void as a temporary workaround for the current erase which can be inefficient because it has to find the next element to return an iterator.

  • Add templated find overload for compatible keys.

  • Ticket 3773: Add missing std qualifier to ptrdiff_t.

  • Some code formatting changes to fit almost all lines into 80 characters.

Release 1.41.0 - Major update

  • The original version made heavy use of macros to sidestep some of the older compilers' poor template support. But since I no longer support those compilers and the macro use was starting to become a maintenance burden it has been rewritten to use templates instead of macros for the implementation classes.

  • The container object is now smaller thanks to using boost::compressed_pair for EBO and a slightly different function buffer - now using a bool instead of a member pointer.

  • Buckets are allocated lazily which means that constructing an empty container will not allocate any memory.

Release 1.40.0

  • Ticket 2975: Store the prime list as a preprocessor sequence - so that it will always get the length right if it changes again in the future.

  • Ticket 1978: Implement emplace for all compilers.

  • Ticket 2908, Ticket 3096: Some workarounds for old versions of borland, including adding explicit destructors to all containers.

  • Ticket 3082: Disable incorrect Visual C++ warnings.

  • Better configuration for C++0x features when the headers aren’t available.

  • Create less buckets by default.

Release 1.39.0

  • Ticket 2756: Avoid a warning on Visual C++ 2009.

  • Some other minor internal changes to the implementation, tests and documentation.

  • Avoid an unnecessary copy in operator[].

  • Ticket 2975: Fix length of prime number list.

Release 1.38.0

  • Use boost::swap.

  • Ticket 2237: Document that the equality and inequality operators are undefined for two objects if their equality predicates aren’t equivalent. Thanks to Daniel Krügler.

  • Ticket 1710: Use a larger prime number list. Thanks to Thorsten Ottosen and Hervé Brönnimann.

  • Use aligned storage to store the types. This changes the way the allocator is used to construct nodes. It used to construct the node with two calls to the allocator’s construct method - once for the pointers and once for the value. It now constructs the node with a single call to construct and then constructs the value using in place construction.

  • Add support for C++0x initializer lists where they’re available (currently only g++ 4.4 in C++0x mode).

Release 1.37.0

  • Rename overload of emplace with hint, to emplace_hint as specified in n2691.

  • Provide forwarding headers at <boost/unordered/unordered_map_fwd.hpp> and <boost/unordered/unordered_set_fwd.hpp>.

  • Move all the implementation inside boost/unordered, to assist modularization and hopefully make it easier to track Release subversion.

Release 1.36.0

First official release.

  • Rearrange the internals.

  • Move semantics - full support when rvalue references are available, emulated using a cut down version of the Adobe move library when they are not.

  • Emplace support when rvalue references and variadic template are available.

  • More efficient node allocation when rvalue references and variadic template are available.

  • Added equality operators.

Boost 1.35.0 Add-on - 31st March 2008

Unofficial release uploaded to vault, to be used with Boost 1.35.0. Incorporated many of the suggestions from the review.

  • Improved portability thanks to Boost regression testing.

  • Fix lots of typos, and clearer text in the documentation.

  • Fix floating point to std::size_t conversion when calculating sizes from the max load factor, and use double in the calculation for greater accuracy.

  • Fix some errors in the examples.

Review Version

Initial review version, for the review conducted from 7th December 2007 to 16th December 2007.

Bibliography

Daniel James

Copyright © 2003, 2004 Jeremy B. Maitin-Shepard

Copyright © 2005-2008 Daniel James

Copyright © 2022-2023 Christian Mazakas

Copyright © 2022-2023 Joaquín M López Muñoz

Copyright © 2022-2023 Peter Dimov

Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)