std::allocator_traits - cppreference.com (original) (raw)

| | | | | -------------------------------------------------- | | ------------- | | 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.

It is possible to use any class type as an allocator, as long as the user-provided specialization of std::allocator_traits implements all required functionality. (until C++23)
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:

Contents

[edit] 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

[edit] Member alias templates

Type Definition
rebind_alloc Alloc::rebind::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 std::allocator_traits<rebind_alloc<T>>

[edit] Member functions

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

[edit] 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

[edit] See also