mfbt/GuardObjects.h

   1 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* This Source Code Form is subject to the terms of the Mozilla Public
   3  * License, v. 2.0. If a copy of the MPL was not distributed with this
   4  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   5
   6 /* Implementation of macros to ensure correct use of RAII Auto* objects. */
   7
   8 #ifndef mozilla_GuardObjects_h
   9 #define mozilla_GuardObjects_h
  10
  11 #include "mozilla/Assertions.h"
  12 #include "mozilla/Types.h"
  13
  14 #ifdef __cplusplus
  15
  16 #ifdef DEBUG
  17
  18 namespace mozilla {
  19 namespace detail {
  20
  21 /*
  22  * The following classes are designed to cause assertions to detect
  23  * inadvertent use of guard objects as temporaries. In other words,
  24  * when we have a guard object whose only purpose is its constructor and
  25  * destructor (and is never otherwise referenced), the intended use
  26  * might be:
  27  *
  28  *   AutoRestore savePainting(mIsPainting);
  29  *
  30  * but is is easy to accidentally write:
  31  *
  32  *   AutoRestore(mIsPainting);
  33  *
  34  * which compiles just fine, but runs the destructor well before the
  35  * intended time.
  36  *
  37  * They work by adding (#ifdef DEBUG) an additional parameter to the
  38  * guard object's constructor, with a default value, so that users of
  39  * the guard object's API do not need to do anything. The default value
  40  * of this parameter is a temporary object. C++ (ISO/IEC 14882:1998),
  41  * section 12.2 [class.temporary], clauses 4 and 5 seem to assume a
  42  * guarantee that temporaries are destroyed in the reverse of their
  43  * construction order, but I actually can't find a statement that that
  44  * is true in the general case (beyond the two specific cases mentioned
  45  * there). However, it seems to be true.
  46  *
  47  * These classes are intended to be used only via the macros immediately
  48  * below them:
  49  *
  50  *   MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER declares (ifdef DEBUG) a member
  51  *     variable, and should be put where a declaration of a private
  52  *     member variable would be placed.
  53  *   MOZ_GUARD_OBJECT_NOTIFIER_PARAM should be placed at the end of the
  54  *     parameters to each constructor of the guard object; it declares
  55  *     (ifdef DEBUG) an additional parameter. (But use the *_ONLY_PARAM
  56  *     variant for constructors that take no other parameters.)
  57  *   MOZ_GUARD_OBJECT_NOTIFIER_PARAM_IN_IMPL should likewise be used in
  58  *     the implementation of such constructors when they are not inline.
  59  *   MOZ_GUARD_OBJECT_NOTIFIER_PARAM_TO_PARENT should be used in
  60  *     the implementation of such constructors to pass the parameter to
  61  *     a base class that also uses these macros
  62  *   MOZ_GUARD_OBJECT_NOTIFIER_INIT is a statement that belongs in each
  63  *     constructor. It uses the parameter declared by
  64  *     MOZ_GUARD_OBJECT_NOTIFIER_PARAM.
  65  *
  66  * For more details, and examples of using these macros, see
  67  * https://developer.mozilla.org/en/Using_RAII_classes_in_Mozilla
  68  */
  69 class MOZ_EXPORT GuardObjectNotifier
  70 {
  71   private:
  72     bool* statementDone;
  73
  74   public:
  75     GuardObjectNotifier() : statementDone(NULL) { }
  76
  77     ~GuardObjectNotifier() {
  78       *statementDone = true;
  79     }
  80
  81     void setStatementDone(bool* statementIsDone) {
  82       statementDone = statementIsDone;
  83     }
  84 };
  85
  86 class MOZ_EXPORT GuardObjectNotificationReceiver
  87 {
  88   private:
  89     bool statementDone;
  90
  91   public:
  92     GuardObjectNotificationReceiver() : statementDone(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(statementDone);
 101     }
 102
 103     void init(const GuardObjectNotifier& constNotifier) {
 104       /*
 105        * constNotifier is passed as a const reference so that we can pass a
 106        * temporary, but we really intend it as non-const.
 107        */
 108       GuardObjectNotifier& notifier = const_cast<GuardObjectNotifier&>(constNotifier);
 109       notifier.setStatementDone(&statementDone);
 110     }
 111 };
 112
 113 } /* namespace detail */
 114 } /* namespace mozilla */
 115
 116 #endif /* DEBUG */
 117
 118 #ifdef DEBUG
 119 #  define MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER \
 120      mozilla::detail::GuardObjectNotificationReceiver _mCheckNotUsedAsTemporary;
 121 #  define MOZ_GUARD_OBJECT_NOTIFIER_PARAM \
 122      , const mozilla::detail::GuardObjectNotifier& _notifier = \
 123          mozilla::detail::GuardObjectNotifier()
 124 #  define MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM \
 125      const mozilla::detail::GuardObjectNotifier& _notifier = \
 126          mozilla::detail::GuardObjectNotifier()
 127 #  define MOZ_GUARD_OBJECT_NOTIFIER_PARAM_IN_IMPL \
 128      , const mozilla::detail::GuardObjectNotifier& _notifier
 129 #  define MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM_IN_IMPL \
 130      const mozilla::detail::GuardObjectNotifier& _notifier
 131 #  define MOZ_GUARD_OBJECT_NOTIFIER_PARAM_TO_PARENT \
 132      , _notifier
 133 #  define MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM_TO_PARENT \
 134        _notifier
 135 #  define MOZ_GUARD_OBJECT_NOTIFIER_INIT \
 136      do { _mCheckNotUsedAsTemporary.init(_notifier); } while (0)
 137 #else
 138 #  define MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
 139 #  define MOZ_GUARD_OBJECT_NOTIFIER_PARAM
 140 #  define MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM
 141 #  define MOZ_GUARD_OBJECT_NOTIFIER_PARAM_IN_IMPL
 142 #  define MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM_IN_IMPL
 143 #  define MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM_TO_PARENT
 144 #  define MOZ_GUARD_OBJECT_NOTIFIER_PARAM_TO_PARENT
 145 #  define MOZ_GUARD_OBJECT_NOTIFIER_INIT do { } while (0)
 146 #endif
 147
 148 #endif /* __cplusplus */
 149
 150 #endif /* mozilla_GuardObjects_h */