1 //===----------------------------------------------------------------------===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 9 #ifndef _LIBCPP___TYPE_TRAITS_IS_REPLACEABLE_H 10 #define _LIBCPP___TYPE_TRAITS_IS_REPLACEABLE_H 11 12 #include <__config> 13 #include <__type_traits/enable_if.h> 14 #include <__type_traits/integral_constant.h> 15 #include <__type_traits/is_same.h> 16 #include <__type_traits/is_trivially_copyable.h> 17 18 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER) 19 # pragma GCC system_header 20 #endif 21 22 _LIBCPP_BEGIN_NAMESPACE_STD 23 24 // A type is replaceable if, with `x` and `y` being different objects, `x = std::move(y)` is equivalent to: 25 // 26 // std::destroy_at(&x) 27 // std::construct_at(&x, std::move(y)) 28 // 29 // This allows turning a move-assignment into a sequence of destroy + move-construct, which 30 // is often more efficient. This is especially relevant when the move-construct is in fact 31 // part of a trivial relocation from somewhere else, in which case there is a huge win. 32 // 33 // Note that this requires language support in order to be really effective, but we 34 // currently emulate the base template with something very conservative. 35 template <class _Tp, class = void> 36 struct __is_replaceable : is_trivially_copyable<_Tp> {}; 37 38 template <class _Tp> 39 struct __is_replaceable<_Tp, __enable_if_t<is_same<_Tp, typename _Tp::__replaceable>::value> > : true_type {}; 40 41 template <class _Tp> 42 inline const bool __is_replaceable_v = __is_replaceable<_Tp>::value; 43 44 // Determines whether an allocator member of a container is replaceable. 45 // 46 // First, we require the allocator type to be considered replaceable. If not, then something fishy might be 47 // happening. Assuming the allocator type is replaceable, we conclude replaceability of the allocator as a 48 // member of the container if the allocator always compares equal (in which case propagation doesn't matter), 49 // or if the allocator always propagates on assignment, which is required in order for move construction and 50 // assignment to be equivalent. 51 template <class _AllocatorTraits> 52 struct __container_allocator_is_replaceable 53 : integral_constant<bool, 54 __is_replaceable_v<typename _AllocatorTraits::allocator_type> && 55 (_AllocatorTraits::is_always_equal::value || 56 (_AllocatorTraits::propagate_on_container_move_assignment::value && 57 _AllocatorTraits::propagate_on_container_copy_assignment::value))> {}; 58 59 _LIBCPP_END_NAMESPACE_STD 60 61 #endif // _LIBCPP___TYPE_TRAITS_IS_REPLACEABLE_H 62