# std::allocator

From cppreference.com
< cpp‎ | memory

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)

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)

### 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++14) | 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++17) | std::true_type

### Member functions

(constructor) | creates a new allocator instance (public member function)
(destructor) | destructs an allocator instance (public member function)
address (deprecated in C++17)(removed in C++20) | 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 (deprecated in C++17)(removed in C++20) | returns the largest supported allocation size (public member function)
construct (deprecated in C++17)(removed in C++20) | constructs an object in allocated storage (public member function)
destroy (deprecated in C++17)(removed in C++20) | destructs an object in allocated storage (public member function)

### Non-member functions

operator==operator!= | 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::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)

### Example

Run this code

    #include <memory>
    #include <iostream>
    #include <string>

    int main()
    {
        std::allocator<int> a1;   // default allocator for ints
        int* a = a1.allocate(1);  // space for one int
        a1.construct(a, 7);       // construct the int
        std::cout << a[0] << '\n';
        a1.deallocate(a, 1);      // deallocate space for one int

        // 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

### 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)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/memory/allocator&oldid=103249"