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comparison: mfbt/GuardObjects.h

mfbt/GuardObjects.h

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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 */

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