std:: allocator_traits

Defined in header <memory>
template < class Alloc > struct allocator_traits ;		(since C++11)

The allocator_traits class template provides the standardized way to access various properties of Allocator s . 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 std::allocator_traits implements all required functionality.

A program that declares an explicit or partial specialization of std::allocator_traits is ill-formed, no diagnostic required.

(since C++23)

The default, non-specialized, std::allocator_traits contains the following members:

Member types

Type	Definition
allocator_type	Alloc
value_type	Alloc::value_type
pointer	Alloc::pointer if present, otherwise value_type*
const_pointer	Alloc::const_pointer if present, otherwise std:: pointer_traits < pointer > :: rebind < const value_type >
void_pointer	Alloc::void_pointer if present, otherwise std:: pointer_traits < pointer > :: rebind < void >
const_void_pointer	Alloc::const_void_pointer if present, otherwise std:: pointer_traits < pointer > :: rebind < const void >
difference_type	Alloc::difference_type if present, otherwise std:: pointer_traits < pointer > :: difference_type
size_type	Alloc::size_type if present, otherwise std:: make_unsigned < difference_type > :: type
propagate_on_container_copy_assignment	Alloc::propagate_on_container_copy_assignment if present, otherwise std::false_type
propagate_on_container_move_assignment	Alloc::propagate_on_container_move_assignment if present, otherwise std::false_type
propagate_on_container_swap	Alloc::propagate_on_container_swap if present, otherwise std::false_type
is_always_equal	Alloc::is_always_equal if present, otherwise std:: is_empty < Alloc > :: type

Member alias templates

Type	Definition
rebind_alloc<T>	Alloc::rebind<T>::other if present, otherwise SomeAllocator<T, Args> if this Alloc is of the form SomeAllocator < U, Args > , where Args is zero or more type arguments
rebind_traits<T>	std :: allocator_traits < rebind_alloc < T >>

Member functions

allocate [static]	allocates uninitialized storage using the allocator (public static member function)
allocate_at_least [static] (C++23)	allocates storage at least as large as the requested size via an allocator (public static member function)
deallocate [static]	deallocates storage using the allocator (public static member function)
construct [static]	constructs an object in the allocated storage (function template)
destroy [static]	destructs an object stored in the allocated storage (function template)
max_size [static]	returns the maximum object size supported by the allocator (public static member function)
select_on_container_copy_construction [static]	obtains the allocator to use after copying a standard container (public static member function)

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2108	C++11	there was no way to show an allocator is stateless	is_always_equal provided

See also

AllocatorAwareContainer (C++11)	container using allocator; associated traits (e.g. propagate_on_container_swap ) usage (named requirement)
allocator	the default allocator (class template)
scoped_allocator_adaptor (C++11)	implements multi-level allocator for multi-level containers (class template)
pointer_traits (C++11)	provides information about pointer-like types (class template)