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* statementDone;
  75
  76   public:
  77     GuardObjectNotifier() : statementDone(nullptr) { }
  78
  79     ~GuardObjectNotifier() {
  80       *statementDone = true;
  81     }
  82
  83     void setStatementDone(bool* statementIsDone) {
  84       statementDone = statementIsDone;
  85     }
  86 };
  87
  88 class GuardObjectNotificationReceiver
  89 {
  90   private:
  91     bool statementDone;
  92
  93   public:
  94     GuardObjectNotificationReceiver() : statementDone(false) { }
  95
  96     ~GuardObjectNotificationReceiver() {
  97       /*
  98        * Assert that the guard object was not used as a temporary.  (Note that
  99        * this assert might also fire if init is not called because the guard
 100        * object's implementation is not using the above macros correctly.)
 101        */
 102       MOZ_ASSERT(statementDone);
 103     }
 104
 105     void init(const GuardObjectNotifier& constNotifier) {
 106       /*
 107        * constNotifier 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 = const_cast<GuardObjectNotifier&>(constNotifier);
 111       notifier.setStatementDone(&statementDone);
 112     }
 113 };
 114
 115 } /* namespace detail */
 116 } /* namespace mozilla */
 117
 118 #endif /* DEBUG */
 119
 120 #ifdef DEBUG
 121 #  define MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER \
 122      mozilla::detail::GuardObjectNotificationReceiver _mCheckNotUsedAsTemporary;
 123 #  define MOZ_GUARD_OBJECT_NOTIFIER_PARAM \
 124      , const mozilla::detail::GuardObjectNotifier& _notifier = \
 125          mozilla::detail::GuardObjectNotifier()
 126 #  define MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM \
 127      const mozilla::detail::GuardObjectNotifier& _notifier = \
 128          mozilla::detail::GuardObjectNotifier()
 129 #  define MOZ_GUARD_OBJECT_NOTIFIER_PARAM_IN_IMPL \
 130      , const mozilla::detail::GuardObjectNotifier& _notifier
 131 #  define MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM_IN_IMPL \
 132      const mozilla::detail::GuardObjectNotifier& _notifier
 133 #  define MOZ_GUARD_OBJECT_NOTIFIER_PARAM_TO_PARENT \
 134      , _notifier
 135 #  define MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM_TO_PARENT \
 136        _notifier
 137 #  define MOZ_GUARD_OBJECT_NOTIFIER_INIT \
 138      do { _mCheckNotUsedAsTemporary.init(_notifier); } while (0)
 139 #else
 140 #  define MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
 141 #  define MOZ_GUARD_OBJECT_NOTIFIER_PARAM
 142 #  define MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM
 143 #  define MOZ_GUARD_OBJECT_NOTIFIER_PARAM_IN_IMPL
 144 #  define MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM_IN_IMPL
 145 #  define MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM_TO_PARENT
 146 #  define MOZ_GUARD_OBJECT_NOTIFIER_PARAM_TO_PARENT
 147 #  define MOZ_GUARD_OBJECT_NOTIFIER_INIT do { } while (0)
 148 #endif
 149
 150 #endif /* __cplusplus */
 151
 152 #endif /* mozilla_GuardObjects_h */