C++ <memory> - allocator_traits class template

The C++ <memory> allocator_traits class template provides the standardized way to access various properties of Allocators. The standard containers and other standard library components access allocators through this template, which makes it possible to use any class type as an allocator, as long as the user-provided specialization of allocator_traits implements all required functionality.

Syntax

C++11

template <class Alloc> struct allocator_traits;

Parameters

Alloc The allocator type, aliased as member type allocator_type.

Member Types

The following aliases are member types of allocator_traits. Any specialization shall also define these member types:

Member types	Definition
allocator_type	the template parameter (Alloc)
value_type	allocator_type::value_type
pointer	allocator_type::pointer (if defined) value_type* (otherwise)
const_pointer	allocator_type::const_pointer (if defined) pointer_traits<pointer>::rebind<const value_type> (otherwise)
void_pointer	allocator_type::void_pointer (if defined) pointer_traits<pointer>::rebind<void> (otherwise)
const_void_pointer	allocator_type::const_void_pointer (if defined) pointer_traits<pointer>::rebind<const void> (otherwise)
difference_type	allocator_type::difference_type (if defined) pointer_traits<pointer>::difference_type (otherwise)
size_type	allocator_type::size_type (if defined) make_unsigned<difference_type>::type (otherwise)
propagate_on_container_copy_assignment	allocator_type::propagate_on_container_copy_assignment (if defined) false_type (otherwise)
propagate_on_container_move_assignment	allocator_type::propagate_on_container_move_assignment (if defined) false_type (otherwise)
propagate_on_container_swap	allocator_type::propagate_on_container_swap (if defined) false_type (otherwise)
rebind_alloc<T>	allocator_type::rebind<T>::other (if defined) AllocTemplate<T,Args> (if Alloc is an instantiation of the form AllocTemplate<U,Args>, where Args are zero or more type arguments) Note: This is an alias template
rebind_traits<T>	allocator_traits<rebind_alloc<T>> Note: This is an alias template

Member Functions

Functions	Description
allocate	Allocate block of storage
deallocate	Release block of storage
construct	Construct an element
destroy	Destroy object
max_size	Maximum size possible to allocate
select_on_container_copy_construction	Select on container copy construction

Example:

The example below shows how to use custom allocator with a standard library container.

#include <cstddef>
#include <iostream>
#include <memory>
#include <vector>

using namespace std;

template <class T>
struct custom_allocator {
  typedef T value_type;
  custom_allocator() noexcept {}
  template <class U> custom_allocator (const custom_allocator<U>&) noexcept {}
  T* allocate (size_t n) {return static_cast<T*>(::operator new(n*sizeof(T))); }
  void deallocate (T* p, size_t n) {::delete(p); }
};

template <class T, class U>
constexpr bool operator== (const custom_allocator<T>&, const custom_allocator<U>&) noexcept
{return true;}

template <class T, class U>
constexpr bool operator!= (const custom_allocator<T>&, const custom_allocator<U>&) noexcept
{return false;}

int main () {
  vector<int,custom_allocator<int>> foo = {100, 200, 300};
  for (auto x: foo) cout<<x<<" ";
  cout<<endl;
  return 0;
}

The output of the above code will be:

100 200 300

❮ C++ <memory> Library