std::allocator
| Defined in | memory |
// 1)
template< class T >
struct allocator;
// 2)
template<>
struct allocator<void>; // 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 (do C++20) The default allocator satisfies allocator completeness requirements (od C++17)
Member types
| pub | Type | Definition |
| pub | value_type | T |
| pub | pointer (removed in C++20) | T* |
| pub | const_pointer (removed in C++20) | const T* |
| pub | reference (removed in C++20) | T& |
| pub | const_reference (removed in C++20) | const T& |
| pub | size_type | std::ptrdiff_t |
| pub | difference_type | std::ptrdiff_t |
| pub | propagate_on_container_move_assignment (od C++11) | std::true_type |
| pub | rebind (removed in C++20) | |
| pub | is_always_equal (od C++11) | std::true_type |
Member function
| pub | (constructor) | creates a new allocator instance |
| pub | (destructor) | destructs an allocator instance |
| pub | address (do C++20) | obtains the address of an object, even if operator& is overloaded |
| pub | allocate | allocates uninitialized storage |
| pub | allocate_at_least (od C++23) | allocates uninitialized storage at least as large as requested size |
| pub | deallocate | deallocates storage |
| pub | max_size (do C++20) | returns the largest supported allocation size |
| pub | construct (do C++20) | constructs an object in allocated storage |
| pub | addrdestroyess (do C++20) | destructs an object in allocated storage |
Non-member functions
| pub | operator== operator!= (removed in C++20) | compares two allocator instances |
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 (do 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 (od 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
#include <memory>
#include <iostream>
#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);
}
7
foo bar
Defect reports
The following behavior-changing defect reports were applied retroactively to previously ished C++ standards.
| pub DR | Applied to | Behavior as ished | Correct behavior |
|---|---|---|---|
| pub 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 |