std::allocator

Defined in header `<memory>`
template< class T > struct allocator;	(1)
template<> struct allocator<void>;	(2)	(deprecated in C++17) (removed in C++20)

The std::allocator class template is the default Allocator used by all standard library containers if no user-specified allocator is provided. The default allocator is stateless, that is, all instances of the given allocator are interchangeable, compare equal and can deallocate memory allocated by any other instance of the same allocator type.

The explicit specialization for void lacks the member typedefs reference, const_reference, size_type and difference_type. This specialization declares no member functions.

(until C++20)

The default allocator satisfies allocator completeness requirements.

(since C++17)

Member types

Type	Definition
`value_type`	`T`
`pointer` (deprecated in C++17)(removed in C++20)	`T*`
`const_pointer` (deprecated in C++17)(removed in C++20)	const T*
`reference` (deprecated in C++17)(removed in C++20)	`T&`
`const_reference` (deprecated in C++17)(removed in C++20)	const T&
`size_type`	std::size_t
`difference_type`	std::ptrdiff_t
`propagate_on_container_move_assignment` (C++11)	std::true_type
`rebind` (deprecated in C++17)(removed in C++20)	template< class U > struct rebind { typedef allocator<U> other; };
`is_always_equal` (C++11)(deprecated in C++23)(removed in C++26)	std::true_type

Member functions

(constructor)	creates a new allocator instance (public member function)
(destructor)	destructs an allocator instance (public member function)
address (until C++20)	obtains the address of an object, even if operator& is overloaded (public member function)
allocate	allocates uninitialized storage (public member function)
allocate_at_least (C++23)	allocates uninitialized storage at least as large as requested size (public member function)
deallocate	deallocates storage (public member function)
max_size (until C++20)	returns the largest supported allocation size (public member function)
construct (until C++20)	constructs an object in allocated storage (public member function)
destroy (until C++20)	destructs an object in allocated storage (public member function)

Non-member functions

operator==operator!=

(removed in C++20)

compares two allocator instances
(public member function)

Notes

The member template class rebind provides a way to obtain an allocator for a different type. For example, std::list<T, A> allocates nodes of some internal type Node<T>, using the allocator A::rebind<Node<T>>::other(until C++11)std::allocator_traits<A>::rebind_alloc<Node<T>>, which is implemented in terms of A::rebind<Node<T>>::other if A is an std::allocator(since C++11).

Member type is_always_equal is deprecated via LWG issue 3170, because it makes custom allocators derived from std::allocator treated as always equal by default. std::allocator_traits<std::allocator<T>>::is_always_equal is not deprecated and its member constant value is true for any T.

Example

Run this code

#include <iostream>
#include <memory>
#include <string>
 
int main()
{
    // default allocator for ints
    std::allocator<int> alloc1;
 
    // demonstrating the few directly usable members
    static_assert(std::is_same_v<int, decltype(alloc1)::value_type>);
    int* p1 = alloc1.allocate(1); // space for one int
    alloc1.deallocate(p1, 1);     // and it is gone
 
    // Even those can be used through traits though, so no need
    using traits_t1 = std::allocator_traits<decltype(alloc1)>; // The matching trait
    p1 = traits_t1::allocate(alloc1, 1);
    traits_t1::construct(alloc1, p1, 7);  // construct the int
    std::cout << *p1 << '\n';
    traits_t1::deallocate(alloc1, p1, 1); // deallocate space for one int
 
    // default allocator for strings
    std::allocator<std::string> alloc2;
    // matching traits
    using traits_t2 = std::allocator_traits<decltype(alloc2)>;
 
    // Rebinding the allocator using the trait for strings gets the same type
    traits_t2::rebind_alloc<std::string> alloc_ = alloc2;
 
    std::string* p2 = traits_t2::allocate(alloc2, 2); // space for 2 strings
 
    traits_t2::construct(alloc2, p2, "foo");
    traits_t2::construct(alloc2, p2 + 1, "bar");
 
    std::cout << p2[0] << ' ' << p2[1] << '\n';
 
    traits_t2::destroy(alloc2, p2 + 1);
    traits_t2::destroy(alloc2, p2);
    traits_t2::deallocate(alloc2, p2, 2);
}

Output:

7
foo bar

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 2103	C++11	redundant comparison between `allocator` might be required	`propagate_on_container_move_assignment` provided
LWG 2108	C++11	there was no way to show `allocator` is stateless	`is_always_equal` provided

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

Member functions
allocator::allocator
allocator::~allocator
allocator::address (until C++20)
allocator::allocate
allocator::allocate_at_least (C++23)
allocator::deallocate
allocator::max_size (until C++20)
allocator::construct (until C++20)
allocator::destroy (until C++20)
Non-member functions
operator==operator!= (until C++20)

allocator_traits (C++11)	provides information about allocator types (class template)
scoped_allocator_adaptor (C++11)	implements multi-level allocator for multi-level containers (class template)
uses_allocator (C++11)	checks if the specified type supports uses-allocator construction (class template)