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