2 * Copyright 2016 Facebook, Inc.
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
8 * http://www.apache.org/licenses/LICENSE-2.0
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
17 #ifndef FOLLY_SCOPEGUARD_H_
18 #define FOLLY_SCOPEGUARD_H_
23 #include <type_traits>
26 #include <folly/Preprocessor.h>
27 #include <folly/detail/UncaughtExceptionCounter.h>
32 * ScopeGuard is a general implementation of the "Initialization is
33 * Resource Acquisition" idiom. Basically, it guarantees that a function
34 * is executed upon leaving the currrent scope unless otherwise told.
36 * The makeGuard() function is used to create a new ScopeGuard object.
37 * It can be instantiated with a lambda function, a std::function<void()>,
38 * a functor, or a void(*)() function pointer.
41 * Usage example: Add a friend to memory iff it is also added to the db.
43 * void User::addFriend(User& newFriend) {
44 * // add the friend to memory
45 * friends_.push_back(&newFriend);
47 * // If the db insertion that follows fails, we should
48 * // remove it from memory.
49 * // (You could also declare this as "auto guard = makeGuard(...)")
50 * ScopeGuard guard = makeGuard([&] { friends_.pop_back(); });
52 * // this will throw an exception upon error, which
53 * // makes the ScopeGuard execute UserCont::pop_back()
54 * // once the Guard's destructor is called.
55 * db_->addFriend(GetName(), newFriend.GetName());
57 * // an exception was not thrown, so don't execute
62 * Examine ScopeGuardTest.cpp for some more sample usage.
65 * Andrei's and Petru Marginean's CUJ article:
66 * http://drdobbs.com/184403758
67 * and the loki library:
68 * http://loki-lib.sourceforge.net/index.php?n=Idioms.ScopeGuardPointer
69 * and triendl.kj article:
70 * http://www.codeproject.com/KB/cpp/scope_guard.aspx
72 class ScopeGuardImplBase {
74 void dismiss() noexcept {
79 ScopeGuardImplBase() noexcept : dismissed_(false) {}
81 static ScopeGuardImplBase makeEmptyScopeGuard() noexcept {
82 return ScopeGuardImplBase{};
86 static const T& asConst(const T& t) noexcept {
93 template <typename FunctionType>
94 class ScopeGuardImpl : public ScopeGuardImplBase {
96 explicit ScopeGuardImpl(FunctionType& fn) noexcept(
97 std::is_nothrow_copy_constructible<FunctionType>::value)
100 makeFailsafe(std::is_nothrow_copy_constructible<FunctionType>{},
103 explicit ScopeGuardImpl(const FunctionType& fn) noexcept(
104 std::is_nothrow_copy_constructible<FunctionType>::value)
107 makeFailsafe(std::is_nothrow_copy_constructible<FunctionType>{},
110 explicit ScopeGuardImpl(FunctionType&& fn) noexcept(
111 std::is_nothrow_move_constructible<FunctionType>::value)
113 std::move_if_noexcept(fn),
114 makeFailsafe(std::is_nothrow_move_constructible<FunctionType>{},
117 ScopeGuardImpl(ScopeGuardImpl&& other) noexcept(
118 std::is_nothrow_move_constructible<FunctionType>::value)
119 : function_(std::move_if_noexcept(other.function_)) {
120 // If the above line attempts a copy and the copy throws, other is
121 // left owning the cleanup action and will execute it (or not) depending
122 // on the value of other.dismissed_. The following lines only execute
123 // if the move/copy succeeded, in which case *this assumes ownership of
124 // the cleanup action and dismisses other.
125 dismissed_ = other.dismissed_;
126 other.dismissed_ = true;
129 ~ScopeGuardImpl() noexcept {
136 static ScopeGuardImplBase makeFailsafe(std::true_type, const void*) noexcept {
137 return makeEmptyScopeGuard();
140 template <typename Fn>
141 static auto makeFailsafe(std::false_type, Fn* fn) noexcept
142 -> ScopeGuardImpl<decltype(std::ref(*fn))> {
143 return ScopeGuardImpl<decltype(std::ref(*fn))>{std::ref(*fn)};
146 template <typename Fn>
147 explicit ScopeGuardImpl(Fn&& fn, ScopeGuardImplBase&& failsafe)
148 : ScopeGuardImplBase{}, function_(std::forward<Fn>(fn)) {
152 void* operator new(std::size_t) = delete;
154 void execute() noexcept { function_(); }
156 FunctionType function_;
159 template <typename FunctionType>
160 ScopeGuardImpl<typename std::decay<FunctionType>::type>
161 makeGuard(FunctionType&& fn) noexcept(
162 std::is_nothrow_constructible<typename std::decay<FunctionType>::type,
163 FunctionType>::value) {
164 return ScopeGuardImpl<typename std::decay<FunctionType>::type>(
165 std::forward<FunctionType>(fn));
169 * This is largely unneeded if you just use auto for your guards.
171 typedef ScopeGuardImplBase&& ScopeGuard;
175 #if defined(FOLLY_EXCEPTION_COUNT_USE_CXA_GET_GLOBALS) || \
176 defined(FOLLY_EXCEPTION_COUNT_USE_GETPTD)
179 * ScopeGuard used for executing a function when leaving the current scope
180 * depending on the presence of a new uncaught exception.
182 * If the executeOnException template parameter is true, the function is
183 * executed if a new uncaught exception is present at the end of the scope.
184 * If the parameter is false, then the function is executed if no new uncaught
185 * exceptions are present at the end of the scope.
187 * Used to implement SCOPE_FAIL and SCOPE_SUCCES below.
189 template <typename FunctionType, bool executeOnException>
190 class ScopeGuardForNewException {
192 explicit ScopeGuardForNewException(const FunctionType& fn)
196 explicit ScopeGuardForNewException(FunctionType&& fn)
197 : function_(std::move(fn)) {
200 ScopeGuardForNewException(ScopeGuardForNewException&& other)
201 : function_(std::move(other.function_))
202 , exceptionCounter_(std::move(other.exceptionCounter_)) {
205 ~ScopeGuardForNewException() noexcept(executeOnException) {
206 if (executeOnException == exceptionCounter_.isNewUncaughtException()) {
212 ScopeGuardForNewException(const ScopeGuardForNewException& other) = delete;
214 void* operator new(std::size_t) = delete;
216 FunctionType function_;
217 UncaughtExceptionCounter exceptionCounter_;
221 * Internal use for the macro SCOPE_FAIL below
223 enum class ScopeGuardOnFail {};
225 template <typename FunctionType>
226 ScopeGuardForNewException<typename std::decay<FunctionType>::type, true>
227 operator+(detail::ScopeGuardOnFail, FunctionType&& fn) {
229 ScopeGuardForNewException<typename std::decay<FunctionType>::type, true>(
230 std::forward<FunctionType>(fn));
234 * Internal use for the macro SCOPE_SUCCESS below
236 enum class ScopeGuardOnSuccess {};
238 template <typename FunctionType>
239 ScopeGuardForNewException<typename std::decay<FunctionType>::type, false>
240 operator+(ScopeGuardOnSuccess, FunctionType&& fn) {
242 ScopeGuardForNewException<typename std::decay<FunctionType>::type, false>(
243 std::forward<FunctionType>(fn));
246 #endif // native uncaught_exception() supported
249 * Internal use for the macro SCOPE_EXIT below
251 enum class ScopeGuardOnExit {};
253 template <typename FunctionType>
254 ScopeGuardImpl<typename std::decay<FunctionType>::type>
255 operator+(detail::ScopeGuardOnExit, FunctionType&& fn) {
256 return ScopeGuardImpl<typename std::decay<FunctionType>::type>(
257 std::forward<FunctionType>(fn));
259 } // namespace detail
264 auto FB_ANONYMOUS_VARIABLE(SCOPE_EXIT_STATE) \
265 = ::folly::detail::ScopeGuardOnExit() + [&]() noexcept
267 #if defined(FOLLY_EXCEPTION_COUNT_USE_CXA_GET_GLOBALS) || \
268 defined(FOLLY_EXCEPTION_COUNT_USE_GETPTD)
270 auto FB_ANONYMOUS_VARIABLE(SCOPE_FAIL_STATE) \
271 = ::folly::detail::ScopeGuardOnFail() + [&]() noexcept
273 #define SCOPE_SUCCESS \
274 auto FB_ANONYMOUS_VARIABLE(SCOPE_SUCCESS_STATE) \
275 = ::folly::detail::ScopeGuardOnSuccess() + [&]()
276 #endif // native uncaught_exception() supported
278 #endif // FOLLY_SCOPEGUARD_H_