cleanup/internal/cleanup.h

0001 // Copyright 2021 The Abseil Authors.
0002 //
0003 // Licensed under the Apache License, Version 2.0 (the "License");
0004 // you may not use this file except in compliance with the License.
0005 // You may obtain a copy of the License at
0006 //
0007 //      https://www.apache.org/licenses/LICENSE-2.0
0008 //
0009 // Unless required by applicable law or agreed to in writing, software
0010 // distributed under the License is distributed on an "AS IS" BASIS,
0011 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
0012 // See the License for the specific language governing permissions and
0013 // limitations under the License.
0014
0015 #ifndef ABSL_CLEANUP_INTERNAL_CLEANUP_H_
0016 #define ABSL_CLEANUP_INTERNAL_CLEANUP_H_
0017
0018 #include <new>
0019 #include <type_traits>
0020 #include <utility>
0021
0022 #include "absl/base/internal/invoke.h"
0023 #include "absl/base/macros.h"
0024 #include "absl/base/thread_annotations.h"
0025 #include "absl/utility/utility.h"
0026
0027 namespace absl {
0028 ABSL_NAMESPACE_BEGIN
0029
0030 namespace cleanup_internal {
0031
0032 struct Tag {};
0033
0034 template <typename Arg, typename... Args>
0035 constexpr bool WasDeduced() {
0036   return (std::is_same<cleanup_internal::Tag, Arg>::value) &&
0037          (sizeof...(Args) == 0);
0038 }
0039
0040 template <typename Callback>
0041 constexpr bool ReturnsVoid() {
0042   return (std::is_same<base_internal::invoke_result_t<Callback>, void>::value);
0043 }
0044
0045 template <typename Callback>
0046 class Storage {
0047  public:
0048   Storage() = delete;
0049
0050   explicit Storage(Callback callback) {
0051     // Placement-new into a character buffer is used for eager destruction when
0052     // the cleanup is invoked or cancelled. To ensure this optimizes well, the
0053     // behavior is implemented locally instead of using an absl::optional.
0054     ::new (GetCallbackBuffer()) Callback(std::move(callback));
0055     is_callback_engaged_ = true;
0056   }
0057
0058   Storage(Storage&& other) {
0059     ABSL_HARDENING_ASSERT(other.IsCallbackEngaged());
0060
0061     ::new (GetCallbackBuffer()) Callback(std::move(other.GetCallback()));
0062     is_callback_engaged_ = true;
0063
0064     other.DestroyCallback();
0065   }
0066
0067   Storage(const Storage& other) = delete;
0068
0069   Storage& operator=(Storage&& other) = delete;
0070
0071   Storage& operator=(const Storage& other) = delete;
0072
0073   void* GetCallbackBuffer() { return static_cast<void*>(+callback_buffer_); }
0074
0075   Callback& GetCallback() {
0076     return *reinterpret_cast<Callback*>(GetCallbackBuffer());
0077   }
0078
0079   bool IsCallbackEngaged() const { return is_callback_engaged_; }
0080
0081   void DestroyCallback() {
0082     is_callback_engaged_ = false;
0083     GetCallback().~Callback();
0084   }
0085
0086   void InvokeCallback() ABSL_NO_THREAD_SAFETY_ANALYSIS {
0087     std::move(GetCallback())();
0088   }
0089
0090  private:
0091   bool is_callback_engaged_;
0092   alignas(Callback) char callback_buffer_[sizeof(Callback)];
0093 };
0094
0095 }  // namespace cleanup_internal
0096
0097 ABSL_NAMESPACE_END
0098 }  // namespace absl
0099
0100 #endif  // ABSL_CLEANUP_INTERNAL_CLEANUP_H_