std::allocator

Defined in header `<memory>`
template< class T > struct allocator;	(1)
template<> struct allocator<void>;	(2)

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.

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

All custom allocators also must be stateless.	(until C++11)
Custom allocators may contain state. Each container or another allocator-aware object stores an instance of the supplied allocator and controls allocator replacement through std::allocator_traits.	(since C++11)
The default allocator satisfies allocator completeness requirements.	(since C++17)

Type	Definition
`value_type`	T
`pointer`	T*
`const_pointer`	const T*
`reference`	T&
`const_reference`	const T&
`size_type`	std::size_t
`difference_type`	std::ptrdiff_t
`propagate_on_container_move_assignment`(C++14)	std::true_type
`rebind`	template< class U > struct rebind { typedef allocator<U> other; };
`is_always_equal`(C++17)	std::true_type

[edit] Member functions

(constructor)	creates a new allocator instance (public member function)
(destructor)	destructs an allocator instance (public member function)
address	obtains the address of an object, even if operator& is overloaded (public member function)
allocate	allocates uninitialized storage (public member function)
deallocate	deallocates storage (public member function)
max_size	returns the largest supported allocation size (public member function)
construct	constructs an object in allocated storage (public member function)
destroy	destructs an object in allocated storage (public member function)

[edit] Non-member functions

operator==operator!=

compares two allocator instances
(public member function)

[edit] 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::list<T, A>` allocates nodes of some internal type `Node<T>`, using the allocator 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)

[edit] Example

Run this code

#include <memory>
#include <iostream>
#include <string>
 
int main()
{
    std::allocator<int> a1; // default allocator for ints
    int* a = a1.allocate(10); // space for 10 ints
 
    a[9] = 7;
 
    std::cout << a[9] << '\n';
 
    a1.deallocate(a, 10);
 
    // default allocator for strings
    std::allocator<std::string> a2;
 
    // same, but obtained by rebinding from the type of a1
    decltype(a1)::rebind<std::string>::other a2_1;
 
    // same, but obtained by rebinding from the type of a1 via allocator_traits
    std::allocator_traits<decltype(a1)>::rebind_alloc<std::string> a2_2;
 
    std::string* s = a2.allocate(2); // space for 2 strings
 
    a2.construct(s, "foo");
    a2.construct(s + 1, "bar");
 
    std::cout << s[0] << ' ' << s[1] << '\n';
 
    a2.destroy(s);
    a2.destroy(s + 1);
    a2.deallocate(s, 2);
}

Output:

7
foo bar

[edit] See also

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)

Language
Standard library headers
Concepts
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Technical Specifications

Member functions
allocator::allocator
allocator::~allocator
allocator::address
allocator::allocate
allocator::deallocate
allocator::max_size
allocator::construct
allocator::destroy
Non-member functions
operator==operator!=