Quelle ScopeExit.h Sprache: C

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* 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 <utility>

#include "mozilla/Attributes.h"

namespace mozilla {

template <typename ExitFunction>
class MOZ_STACK_CLASS ScopeExit {
  ExitFunction mExitFunction;
  bool mExecuteOnDestruction;

public:
  explicit ScopeExit(ExitFunction&& cleanup)
      : mExitFunction(std::move(cleanup)), mExecuteOnDestruction(true) {}

  ScopeExit(ScopeExit&& rhs)
      : mExitFunction(std::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>
[[nodiscard]] ScopeExit<ExitFunction> MakeScopeExit(
    ExitFunction&& exitFunction) {
  return ScopeExit<ExitFunction>(std::move(exitFunction));
}

} /* namespace mozilla */

#endif /* mozilla_ScopeExit_h */

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