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