std::allocator

 

C++

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

 

Dynamic memory management

Smart pointers
unique_ptr (C++11)
shared_ptr (C++11)
weak_ptr (C++11)
auto_ptr (until C++17)
owner_less (C++11)
enable_shared_from_this (C++11)
bad_weak_ptr (C++11)
default_delete (C++11)
Allocators
allocator
allocator_traits (C++11)
allocator_arg_t (C++11)
allocator_arg (C++11)
uses_allocator (C++11)
scoped_allocator_adaptor (C++11)
Uninitialized storage
uninitialized_copy
uninitialized_copy_n (C++11)
uninitialized_fill
uninitialized_fill_n
raw_storage_iterator
get_temporary_buffer
return_temporary_buffer
Garbage collection support
declare_reachable (C++11)
undeclare_reachable (C++11)
declare_no_pointers (C++11)
undeclare_no_pointers (C++11)
pointer_safety (C++11)
get_pointer_safety (C++11)
Miscellaneous
pointer_traits (C++11)
addressof (C++11)
align (C++11)
C Library
malloc calloc	realloc free
Low level memory management

 

std::allocator

Member functions

allocator::allocator

allocator::~allocator

allocator::address

allocator::allocate

allocator::deallocate

allocator::max_size

allocator::construct

allocator::destroy

Non-member functions

operator==operator!=

 

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)

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

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)

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(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

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)