[libcxx-commits] [libcxx] [libc++] Add the __is_replaceable type trait (PR #132408)

[Nikolas Klauser via libcxx-commits]

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@@ -0,0 +1,62 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___TYPE_TRAITS_IS_REPLACEABLE_H
+#define _LIBCPP___TYPE_TRAITS_IS_REPLACEABLE_H
+
+#include <__config>
+#include <__type_traits/enable_if.h>
+#include <__type_traits/integral_constant.h>
+#include <__type_traits/is_same.h>
+#include <__type_traits/is_trivially_copyable.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+// A type is replaceable if, with `x` and `y` being different objects, `x = std::move(y)` is equivalent to:
+//
+//  std::destroy_at(&x)
+//  std::construct_at(&x, std::move(y))
+//
+// This allows turning a move-assignment into a sequence of destroy + move-construct, which
+// is often more efficient. This is especially relevant when the move-construct is in fact
+// part of a trivial relocation from somewhere else, in which case there is a huge win.
+//
+// Note that this requires language support in order to be really effective, but we
+// currently emulate the base template with something very conservative.
+template <class _Tp, class = void>
+struct __is_replaceable : is_trivially_copyable<_Tp> {};
+
+template <class _Tp>
+struct __is_replaceable<_Tp, __enable_if_t<is_same<_Tp, typename _Tp::__replaceable>::value> > : true_type {};
+
+template <class _Tp>
+inline const bool __is_replaceable_v = __is_replaceable<_Tp>::value;
+
+// Determines whether an allocator member of a container is replaceable.
+//
+// We take into account whether the allocator is propagated on assignments. If the allocator
+// always compares equal, then it doesn't matter whether we propagate it or not on assignments,
+// the result will be the same and we can just as much move-construct it instead.
+//
+// If the allocator does not always compare equal, we check whether it propagates on assignment
+// and it is replaceable.
+template <class _AllocatorTraits>
+struct __container_allocator_is_replaceable
+    : integral_constant<bool,
+                        _AllocatorTraits::is_always_equal::value ||
+                            (_AllocatorTraits::propagate_on_container_move_assignment::value &&
+                             _AllocatorTraits::propagate_on_container_copy_assignment::value &&
+                             __is_replaceable_v<typename _AllocatorTraits::allocator_type>)> {};
----------------
philnik777 wrote:

This looks a bit fishy to me. I'd rather just test the allocator for replacability. If it always compares equal, it's very likely that it's replacable anyways and this logic seems like something we could easily shoot ourselves with. If this were to ever come up in real life we can still add this logic later.

https://github.com/llvm/llvm-project/pull/132408