mfbt/ScopeExit.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 /* RAII class for executing arbitrary actions at scope end. */
   8
   9 #ifndef mozilla_ScopeExit_h
  10 #define mozilla_ScopeExit_h
  11
  12 /*
  13  * See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4189.pdf for a
  14  * standards-track version of this.
  15  *
  16  * Error handling can be complex when various actions need to be performed that
  17  * need to be undone if an error occurs midway. This can be handled with a
  18  * collection of boolean state variables and gotos, which can get clunky and
  19  * error-prone:
  20  *
  21  * {
  22  *   if (!a.setup())
  23  *       goto fail;
  24  *   isASetup = true;
  25  *
  26  *   if (!b.setup())
  27  *       goto fail;
  28  *   isBSetup = true;
  29  *
  30  *   ...
  31  *   return true;
  32  *
  33  *   fail:
  34  *     if (isASetup)
  35  *         a.teardown();
  36  *     if (isBSetup)
  37  *         b.teardown();
  38  *     return false;
  39  *  }
  40  *
  41  * ScopeExit is a mechanism to simplify this pattern by keeping an RAII guard
  42  * class that will perform the teardown on destruction, unless released. So the
  43  * above would become:
  44  *
  45  * {
  46  *   if (!a.setup()) {
  47  *       return false;
  48  *   }
  49  *   auto guardA = MakeScopeExit([&] {
  50  *       a.teardown();
  51  *   });
  52  *
  53  *   if (!b.setup()) {
  54  *       return false;
  55  *   }
  56  *   auto guardB = MakeScopeExit([&] {
  57  *       b.teardown();
  58  *   });
  59  *
  60  *   ...
  61  *   guardA.release();
  62  *   guardB.release();
  63  *   return true;
  64  * }
  65  *
  66  * This header provides:
  67  *
  68  * - |ScopeExit| - a container for a cleanup call, automically called at the
  69  *   end of the scope;
  70  * - |MakeScopeExit| - a convenience function for constructing a |ScopeExit|
  71  *   with a given cleanup routine, commonly used with a lambda function.
  72  *
  73  * Note that the RAII classes defined in this header do _not_ perform any form
  74  * of reference-counting or garbage-collection. These classes have exactly two
  75  * behaviors:
  76  *
  77  * - if |release()| has not been called, the cleanup is always performed at
  78  *   the end of the scope;
  79  * - if |release()| has been called, nothing will happen at the end of the
  80  *   scope.
  81  */
  82
  83 #include <utility>
  84
  85 #include "mozilla/Attributes.h"
  86
  87 namespace mozilla {
  88
  89 template <typename ExitFunction>
  90 class MOZ_STACK_CLASS ScopeExit {
  91   ExitFunction mExitFunction;
  92   bool mExecuteOnDestruction;
  93
  94  public:
  95   explicit ScopeExit(ExitFunction&& cleanup)
  96       : mExitFunction(std::move(cleanup)), mExecuteOnDestruction(true) {}
  97
  98   ScopeExit(ScopeExit&& rhs)
  99       : mExitFunction(std::move(rhs.mExitFunction)),
 100         mExecuteOnDestruction(rhs.mExecuteOnDestruction) {
 101     rhs.release();
 102   }
 103
 104   ~ScopeExit() {
 105     if (mExecuteOnDestruction) {
 106       mExitFunction();
 107     }
 108   }
 109
 110   void release() { mExecuteOnDestruction = false; }
 111
 112  private:
 113   explicit ScopeExit(const ScopeExit&) = delete;
 114   ScopeExit& operator=(const ScopeExit&) = delete;
 115   ScopeExit& operator=(ScopeExit&&) = delete;
 116 };
 117
 118 template <typename ExitFunction>
 119 [[nodiscard]] ScopeExit<ExitFunction> MakeScopeExit(
 120     ExitFunction&& exitFunction) {
 121   return ScopeExit<ExitFunction>(std::move(exitFunction));
 122 }
 123
 124 } /* namespace mozilla */
 125
 126 #endif /* mozilla_ScopeExit_h */