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127 lines
3.3 KiB
C++
127 lines
3.3 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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/* RAII class for executing arbitrary actions at scope end. */
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#ifndef mozilla_ScopeExit_h
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#define mozilla_ScopeExit_h
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/*
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* See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4189.pdf for a
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* standards-track version of this.
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*
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* Error handling can be complex when various actions need to be performed that
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* need to be undone if an error occurs midway. This can be handled with a
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* collection of boolean state variables and gotos, which can get clunky and
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* error-prone:
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*
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* {
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* if (!a.setup())
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* goto fail;
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* isASetup = true;
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*
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* if (!b.setup())
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* goto fail;
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* isBSetup = true;
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*
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* ...
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* return true;
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*
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* fail:
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* if (isASetup)
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* a.teardown();
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* if (isBSetup)
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* b.teardown();
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* return false;
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* }
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*
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* ScopeExit is a mechanism to simplify this pattern by keeping an RAII guard
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* class that will perform the teardown on destruction, unless released. So the
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* above would become:
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*
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* {
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* if (!a.setup()) {
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* return false;
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* }
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* auto guardA = MakeScopeExit([&] {
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* a.teardown();
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* });
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*
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* if (!b.setup()) {
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* return false;
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* }
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* auto guardB = MakeScopeExit([&] {
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* b.teardown();
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* });
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*
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* ...
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* guardA.release();
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* guardB.release();
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* return true;
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* }
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*
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* This header provides:
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*
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* - |ScopeExit| - a container for a cleanup call, automically called at the
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* end of the scope;
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* - |MakeScopeExit| - a convenience function for constructing a |ScopeExit|
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* with a given cleanup routine, commonly used with a lambda function.
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*
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* Note that the RAII classes defined in this header do _not_ perform any form
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* of reference-counting or garbage-collection. These classes have exactly two
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* behaviors:
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*
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* - if |release()| has not been called, the cleanup is always performed at
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* the end of the scope;
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* - if |release()| has been called, nothing will happen at the end of the
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* scope.
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*/
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#include <utility>
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#include "mozilla/Attributes.h"
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namespace mozilla {
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template <typename ExitFunction>
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class MOZ_STACK_CLASS ScopeExit {
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ExitFunction mExitFunction;
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bool mExecuteOnDestruction;
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public:
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explicit ScopeExit(ExitFunction&& cleanup)
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: mExitFunction(std::move(cleanup)), mExecuteOnDestruction(true) {}
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ScopeExit(ScopeExit&& rhs)
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: mExitFunction(std::move(rhs.mExitFunction)),
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mExecuteOnDestruction(rhs.mExecuteOnDestruction) {
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rhs.release();
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}
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~ScopeExit() {
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if (mExecuteOnDestruction) {
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mExitFunction();
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}
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}
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void release() { mExecuteOnDestruction = false; }
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private:
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explicit ScopeExit(const ScopeExit&) = delete;
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ScopeExit& operator=(const ScopeExit&) = delete;
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ScopeExit& operator=(ScopeExit&&) = delete;
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};
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template <typename ExitFunction>
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[[nodiscard]] ScopeExit<ExitFunction> MakeScopeExit(
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ExitFunction&& exitFunction) {
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return ScopeExit<ExitFunction>(std::move(exitFunction));
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}
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} /* namespace mozilla */
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#endif /* mozilla_ScopeExit_h */
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