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| 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 |
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