...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
The intent of Boost.Unordered is to implement a close (but imperfect) implementation of the C++17 standard, that will work with C++98 upwards. The wide compatibility does mean some comprimises have to be made. With a compiler and library that fully support C++11, the differences should be minor.
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:
emplace
,
or if they support Boost.Move, moved into place.
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.
The following support is required for full use of C++11 style construction/destruction:
std::pair
.
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
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.
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.
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.