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std::allocator(3) C++ Standard Libary std::allocator(3) NAME std::allocator - std::allocator Synopsis Defined in header <memory> template< class T > (1) struct allocator; template<> (2) (deprecated in C++17) struct allocator<void>; (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 in- terchangeable, compare equal and can deallocate memory allocated by any other in- stance of the same allocator type. The explicit specialization for void lacks the member typedefs reference, const_reference, size_type and difference_type. This (until C++20) specialization declares no member functions. 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++11) std::true_type template< class U > struct rebind { rebind (deprecated in C++17)(removed in C++20) typedef al- locator<U> other; }; is_always_equal(C++11)(deprecated in C++23) 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 opera- tor& is overloaded (until C++20) (public member function) allocate allocates uninitialized storage (public member function) allocate_at_least allocates uninitialized storage at least as large as requested (C++23) size (public member function) deallocate deallocates storage (public member function) max_size returns the largest supported allocation size (until C++20) (public member function) construct constructs an object in allocated storage (until C++20) (public member function) destroy destructs an object in allocated storage (until C++20) (public member function) Non-member functions operator== compares two allocator instances operator!= (public member function) (removed in C++20) Notes The member template class rebind provides a way to obtain an alloca- tor 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 imple- mented 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, be- cause 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 // Run this code #include <memory> #include <iostream> #include <string> int main() { { // default allocator for ints std::allocator<int> alloc; // demonstrating the few directly usable members static_assert(std::is_same_v<int, decltype(al- loc)::value_type>); int* p = alloc.allocate(1); // space for one int alloc.deallocate(p, 1); // and it is gone // Even those can be used through traits though, so no need using traits_t = std::allocator_traits<decltype(alloc)>; // The matching trait p = traits_t::allocate(alloc, 1); traits_t::construct(alloc, p, 7); // construct the int std::cout << *p << '\n'; traits_t::deallocate(alloc, p, 1); // deallocate space for one int } { // default allocator for strings std::allocator<std::string> alloc; // matching traits using traits_t = std::allocator_traits<decltype(alloc)>; // Rebinding the allocator using the trait for strings gets the same type traits_t::rebind_alloc<std::string> alloc_ = alloc; std::string* p = traits_t::allocate(alloc, 2); // space for 2 strings traits_t::construct(alloc, p, "foo"); traits_t::construct(alloc, p + 1, "bar"); std::cout << p[0] << ' ' << p[1] << '\n'; traits_t::destroy(alloc, p + 1); traits_t::destroy(alloc, p); traits_t::deallocate(alloc, p, 2); } } Output: 7 foo bar Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Correct be- havior redundant comparison propagate_on_con- tainer_move_assignment LWG 2103 C++11 between allocator might provided be required LWG 2108 C++11 there was no way to show is_always_equal pro- vided allocator is stateless See also allocator_traits provides information about allocator types (C++11) (class template) scoped_allocator_adaptor implements multi-level allocator for multi- level containers (C++11) (class template) uses_allocator checks if the specified type supports uses- allocator (C++11) construction (class template) http://cppreference.com 2022.07.31 std::allocator(3)
NAME | Synopsis | Member types | Member functions | Non-member functions | Notes | Example | Output: | See also
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