template< class T >
struct allocator;
template<>
struct allocator<void>;
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.
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++11) 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++11)(deprecated in C++23)(removed in C++26) std::true_type [edit] Member functions creates a new allocator instance
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::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).
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.
#include <iostream>
#include <memory>
#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);
}
Output:
[edit] Defect reportsThe following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior LWG 2103 C++11 redundant comparison betweenallocator 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 [edit] See also
RetroSearch is an open source project built by @garambo | Open a GitHub Issue
Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo
HTML:
3.2
| Encoding:
UTF-8
| Version:
0.7.4