mfbt/GuardObjects.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 /* Implementation of macros to ensure correct use of RAII Auto* objects. */
   8
   9 #ifndef mozilla_GuardObjects_h
  10 #define mozilla_GuardObjects_h
  11
  12 #include "mozilla/Assertions.h"
  13 #include "mozilla/NullPtr.h"
  14 #include "mozilla/Types.h"
  15
  16 #ifdef __cplusplus
  17
  18 #ifdef DEBUG
  19
  20 namespace mozilla {
  21 namespace detail {
  22
  23 /*
  24  * The following classes are designed to cause assertions to detect
  25  * inadvertent use of guard objects as temporaries. In other words,
  26  * when we have a guard object whose only purpose is its constructor and
  27  * destructor (and is never otherwise referenced), the intended use
  28  * might be:
  29  *
  30  *   AutoRestore savePainting(mIsPainting);
  31  *
  32  * but is is easy to accidentally write:
  33  *
  34  *   AutoRestore(mIsPainting);
  35  *
  36  * which compiles just fine, but runs the destructor well before the
  37  * intended time.
  38  *
  39  * They work by adding (#ifdef DEBUG) an additional parameter to the
  40  * guard object's constructor, with a default value, so that users of
  41  * the guard object's API do not need to do anything. The default value
  42  * of this parameter is a temporary object. C++ (ISO/IEC 14882:1998),
  43  * section 12.2 [class.temporary], clauses 4 and 5 seem to assume a
  44  * guarantee that temporaries are destroyed in the reverse of their
  45  * construction order, but I actually can't find a statement that that
  46  * is true in the general case (beyond the two specific cases mentioned
  47  * there). However, it seems to be true.
  48  *
  49  * These classes are intended to be used only via the macros immediately
  50  * below them:
  51  *
  52  *   MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER declares (ifdef DEBUG) a member
  53  *     variable, and should be put where a declaration of a private
  54  *     member variable would be placed.
  55  *   MOZ_GUARD_OBJECT_NOTIFIER_PARAM should be placed at the end of the
  56  *     parameters to each constructor of the guard object; it declares
  57  *     (ifdef DEBUG) an additional parameter. (But use the *_ONLY_PARAM
  58  *     variant for constructors that take no other parameters.)
  59  *   MOZ_GUARD_OBJECT_NOTIFIER_PARAM_IN_IMPL should likewise be used in
  60  *     the implementation of such constructors when they are not inline.
  61  *   MOZ_GUARD_OBJECT_NOTIFIER_PARAM_TO_PARENT should be used in
  62  *     the implementation of such constructors to pass the parameter to
  63  *     a base class that also uses these macros
  64  *   MOZ_GUARD_OBJECT_NOTIFIER_INIT is a statement that belongs in each
  65  *     constructor. It uses the parameter declared by
  66  *     MOZ_GUARD_OBJECT_NOTIFIER_PARAM.
  67  *
  68  * For more details, and examples of using these macros, see
  69  * https://developer.mozilla.org/en/Using_RAII_classes_in_Mozilla
  70  */
  71 class GuardObjectNotifier
  72 {
  73 private:
  74   bool* mStatementDone;
  75
  76 public:
  77   GuardObjectNotifier() : mStatementDone(nullptr) { }
  78
  79   ~GuardObjectNotifier() { *mStatementDone = true; }
  80
  81   void setStatementDone(bool* aStatementIsDone)
  82   {
  83     mStatementDone = aStatementIsDone;
  84   }
  85 };
  86
  87 class GuardObjectNotificationReceiver
  88 {
  89 private:
  90   bool mStatementDone;
  91
  92 public:
  93   GuardObjectNotificationReceiver() : mStatementDone(false) { }
  94
  95   ~GuardObjectNotificationReceiver() {
  96     /*
  97      * Assert that the guard object was not used as a temporary.  (Note that
  98      * this assert might also fire if init is not called because the guard
  99      * object's implementation is not using the above macros correctly.)
 100      */
 101     MOZ_ASSERT(mStatementDone);
 102   }
 103
 104   void init(const GuardObjectNotifier& aConstNotifier)
 105   {
 106     /*
 107      * aConstNotifier is passed as a const reference so that we can pass a
 108      * temporary, but we really intend it as non-const.
 109      */
 110     GuardObjectNotifier& notifier =
 111       const_cast<GuardObjectNotifier&>(aConstNotifier);
 112     notifier.setStatementDone(&mStatementDone);
 113   }
 114 };
 115
 116 } /* namespace detail */
 117 } /* namespace mozilla */
 118
 119 #endif /* DEBUG */
 120
 121 #ifdef DEBUG
 122 #  define MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER \
 123      mozilla::detail::GuardObjectNotificationReceiver _mCheckNotUsedAsTemporary;
 124 #  define MOZ_GUARD_OBJECT_NOTIFIER_PARAM \
 125      , const mozilla::detail::GuardObjectNotifier& _notifier = \
 126          mozilla::detail::GuardObjectNotifier()
 127 #  define MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM \
 128      const mozilla::detail::GuardObjectNotifier& _notifier = \
 129          mozilla::detail::GuardObjectNotifier()
 130 #  define MOZ_GUARD_OBJECT_NOTIFIER_PARAM_IN_IMPL \
 131      , const mozilla::detail::GuardObjectNotifier& _notifier
 132 #  define MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM_IN_IMPL \
 133      const mozilla::detail::GuardObjectNotifier& _notifier
 134 #  define MOZ_GUARD_OBJECT_NOTIFIER_PARAM_TO_PARENT \
 135      , _notifier
 136 #  define MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM_TO_PARENT \
 137        _notifier
 138 #  define MOZ_GUARD_OBJECT_NOTIFIER_INIT \
 139      do { _mCheckNotUsedAsTemporary.init(_notifier); } while (0)
 140 #else
 141 #  define MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
 142 #  define MOZ_GUARD_OBJECT_NOTIFIER_PARAM
 143 #  define MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM
 144 #  define MOZ_GUARD_OBJECT_NOTIFIER_PARAM_IN_IMPL
 145 #  define MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM_IN_IMPL
 146 #  define MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM_TO_PARENT
 147 #  define MOZ_GUARD_OBJECT_NOTIFIER_PARAM_TO_PARENT
 148 #  define MOZ_GUARD_OBJECT_NOTIFIER_INIT do { } while (0)
 149 #endif
 150
 151 #endif /* __cplusplus */
 152
 153 #endif /* mozilla_GuardObjects_h */