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std::allocator

From cppreference.com
< cpp‎ | memory
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)

Contents

[edit] Member types

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

creates a new allocator instance
(public member function) [edit]
destructs an allocator instance
(public member function) [edit]
obtains the address of an object, even if operator& is overloaded
(public member function) [edit]
allocates uninitialized storage
(public member function) [edit]
deallocates storage
(public member function) [edit]
returns the largest supported allocation size
(public member function) [edit]
constructs an object in allocated storage
(public member function) [edit]
destructs an object in allocated storage
(public member function) [edit]

[edit] Non-member functions

compares two allocator instances
(public member function) [edit]

[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

#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 string
 
    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

provides information about allocator types
(class template) [edit]
implements multi-level allocator for multi-level containers
(class template) [edit]
checks if the specified type supports uses-allocator construction
(class template) [edit]