Class template polymorphic_allocator

boost::container::pmr::polymorphic_allocator

Synopsis

// In header: <boost/container/pmr/polymorphic_allocator.hpp>

template<typename T> 
class polymorphic_allocator {
public:
  // types
  typedef T value_type;

  // construct/copy/destruct
  polymorphic_allocator() noexcept;
  polymorphic_allocator(memory_resource *) noexcept;
  polymorphic_allocator(const polymorphic_allocator &) noexcept;
  template<typename U> 
    polymorphic_allocator(const polymorphic_allocator< U > &) noexcept;
  polymorphic_allocator & operator=(const polymorphic_allocator &) noexcept;

  // public member functions
  T * allocate(size_t);
  void deallocate(T *, size_t) noexcept;
  template<typename U, class ... Args> void construct(U *, Args &&...);
  template<typename U> void destroy(U *);
  polymorphic_allocator select_on_container_copy_construction() const noexcept;
  memory_resource * resource() const noexcept;
};

Description

A specialization of class template polymorphic_allocator conforms to the Allocator requirements. Constructed with different memory resources, different instances of the same specialization of polymorphic_allocator can exhibit entirely different allocation behavior. This runtime polymorphism allows objects that use polymorphic_allocator to behave as if they used different allocator types at run time even though they use the same static allocator type.

`polymorphic_allocator` public construct/copy/destruct

```
polymorphic_allocator() noexcept;
```
Effects: Sets m_resource to get_default_resource().
```
polymorphic_allocator(memory_resource * r) noexcept;
```
Requires: r is non-null.

Effects: Sets m_resource to r.

Throws: Nothing

Notes: This constructor provides an implicit conversion from memory_resource*.

polymorphic_allocator(const polymorphic_allocator & other) noexcept;

Effects: Sets m_resource to other.resource().

template<typename U> 
  polymorphic_allocator(const polymorphic_allocator< U > & other) noexcept;

Effects: Sets m_resource to other.resource().

polymorphic_allocator & 
operator=(const polymorphic_allocator & other) noexcept;

Effects: Sets m_resource to other.resource().

`polymorphic_allocator` public member functions

```
T * allocate(size_t n);
```
Returns: Equivalent to static_cast<T*>(m_resource->allocate(n * sizeof(T), alignof(T))).
```
void deallocate(T * p, size_t n) noexcept;
```
Requires: p was allocated from a memory resource, x, equal to *m_resource, using x.allocate(n * sizeof(T), alignof(T)).

Effects: Equivalent to m_resource->deallocate(p, n * sizeof(T), alignof(T)).

Throws: Nothing.
```
template<typename U, class ... Args> void construct(U * p, Args &&... args);
```
Requires: Uses-allocator construction of T with allocator *this and constructor arguments std::forward<Args>(args)... is well-formed. [Note: uses-allocator construction is always well formed for types that do not use allocators. - end note]

Effects: Construct a T object at p by uses-allocator construction with allocator *this and constructor arguments std::forward<Args>(args)....

Throws: Nothing unless the constructor for T throws.

template<typename U> void destroy(U * p);

Effects: p->~U().

polymorphic_allocator select_on_container_copy_construction() const noexcept;

Returns: Equivalent to polymorphic_allocator().

memory_resource * resource() const noexcept;

Returns: m_resource.

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