std::allocator_traits

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Dynamic memory management

Uninitialized memory algorithms uninitialized_copy uninitialized_fill uninitialized_move (C++17) uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) destroy (C++17) destroy_at (C++17) voidify (C++20) uninitialized_copy_n (C++11) uninitialized_fill_n uninitialized_move_n (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17) destroy_n (C++17) construct_at (C++20) Constrained uninitialized memory algorithms ranges::uninitialized_copy (C++20) ranges::uninitialized_fill (C++20) ranges::uninitialized_move (C++20) ranges::construct_at (C++20) ranges::destroy (C++20) ranges::destroy_n (C++20) ranges::destroy_at (C++20) ranges::uninitialized_copy_n (C++20) ranges::uninitialized_fill_n (C++20) ranges::uninitialized_move_n (C++20) ranges::uninitialized_default_construct (C++20) ranges::uninitialized_value_construct (C++20) ranges::uninitialized_default_construct_n (C++20)   ranges::uninitialized_value_construct_n (C++20) Allocators allocator allocator_traits (C++11) allocation_result (C++23) allocator_arg (C++11) uses_allocator (C++11) uses_allocator_construction_args (C++20) make_obj_using_allocator (C++20) uninitialized_construct_using_allocator (C++20) scoped_allocator_adaptor (C++11) pmr::polymorphic_allocator (C++17) Memory resources pmr::memory_resource (C++17) pmr::get_default_resource (C++17) pmr::set_default_resource (C++17) pmr::new_delete_resource (C++17) pmr::null_memory_resource (C++17) pmr::synchronized_pool_resource (C++17) pmr::unsynchronized_pool_resource (C++17)   pmr::monotonic_buffer_resource (C++17) pmr::pool_options (C++17) Garbage collection support declare_reachable (C++11)(until C++23) undeclare_reachable (C++11)(until C++23) declare_no_pointers (C++11)(until C++23) undeclare_no_pointers (C++11)(until C++23) pointer_safety (C++11)(until C++23) get_pointer_safety (C++11)(until C++23) Uninitialized storage raw_storage_iterator (until C++20*) get_temporary_buffer (until C++20*) return_temporary_buffer (until C++20*) Explicit lifetime management start_lifetime_asstart_lifetime_as_array (C++23)(C++23)

Smart pointers unique_ptr (C++11) shared_ptr (C++11) weak_ptr (C++11) auto_ptr (until C++17*) owner_less (C++11) owner_less<void> (C++17) owner_hash (C++26) owner_equal (C++26) enable_shared_from_this (C++11) bad_weak_ptr (C++11) default_delete (C++11) out_ptr_t (C++23) inout_ptr_t (C++23) Low level memory management operator new operator new[] operator delete operator delete[] get_new_handler (C++11) set_new_handler launder (C++17) bad_alloc bad_array_new_length (C++11) nothrow_t align_val_t (C++17) destroying_delete_t (C++20) new_handler nothrow Miscellaneous pointer_traits (C++11) to_address (C++20) addressof (C++11) align (C++11) assume_aligned (C++20) C Library malloc calloc realloc    aligned_alloc (C++17) free

 

std::allocator_traits

Member functions

allocator_traits::allocate (C++11)

allocator_traits::allocate_at_least (C++23)

allocator_traits::deallocate (C++11)

allocator_traits::construct (C++11)

allocator_traits::destroy (C++11)

allocator_traits::max_size (C++11)

allocator_traits::select_on_container_copy_construction (C++11)

 

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 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 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)