std::allocator_traits

template <class Alloc> struct allocator_traits;

Allocator traits

This template supplies a uniform interface for allocator types.

The non-specialized version provides an interface that allows to use as allocator just any class that provides at least a public member type value_type and public member functions allocate and deallocate (see example).

But any class type can be used as allocator by specializing this class to give it an interface.

Template 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 type	definition in unspecialized `allocator_traits`
`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

Static member functions

allocate: Allocate block of storage (public static member function)

deallocate: Release block of storage (public static member function)

construct: Construct an element (public static member function)

destroy: Destroy object (public static member function)

max_size: Maximum size possible to allocate (public static member function)

select_on_container_copy_construction: Select on container copy construction (public static member function)

Example

// custom allocator example
#include <cstddef>
#include <iostream>
#include <memory>
#include <vector>
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 (std::size_t n) { return static_cast<T*>(::operator new(n*sizeof(T))); }
 void deallocate (T* p, std::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 () {
 std::vector<int,custom_allocator<int>> foo = {10,20,30};
 for (auto x: foo) std::cout << x << " ";
 std::cout << '\n';
 return 0;
}

Possible output:

10 20 30

classes

enum classes

functions

objects

static member functions

std::allocator_traits

Template parameters

Member types

Static member functions

Example