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comparison: xpcom/glue/nsTWeakRef.h
xpcom/glue/nsTWeakRef.h
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| 1 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */ | |
| 2 /* vim:set ts=2 sw=2 sts=2 et cindent: */ | |
| 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 #ifndef nsTWeakRef_h__ | |
| 8 #define nsTWeakRef_h__ | |
| 9 | |
| 10 #ifndef nsDebug_h___ | |
| 11 #include "nsDebug.h" | |
| 12 #endif | |
| 13 | |
| 14 /** | |
| 15 * A weak reference class for use with generic C++ objects. NOT THREADSAFE! | |
| 16 * | |
| 17 * Example usage: | |
| 18 * | |
| 19 * class A { | |
| 20 * public: | |
| 21 * A() : mWeakSelf(this) { | |
| 22 * } | |
| 23 * ~A() { | |
| 24 * mWeakSelf.forget(); | |
| 25 * } | |
| 26 * void Bar() { printf("Bar!\n"); } | |
| 27 * const nsTWeakRef<A> &AsWeakRef() const { return mWeakSelf; } | |
| 28 * private: | |
| 29 * nsTWeakRef<A> mWeakSelf; | |
| 30 * }; | |
| 31 * | |
| 32 * class B { | |
| 33 * public: | |
| 34 * void SetA(const nsTWeakRef<A> &a) { | |
| 35 * mA = a; | |
| 36 * } | |
| 37 * void Foo() { | |
| 38 * if (mA) | |
| 39 * mA->Bar(); | |
| 40 * } | |
| 41 * private: | |
| 42 * nsTWeakRef<A> mA; | |
| 43 * }; | |
| 44 * | |
| 45 * void Test() { | |
| 46 * B b; | |
| 47 * { | |
| 48 * A a; | |
| 49 * b.SetA(a.AsWeakRef()); | |
| 50 * b.Foo(); // prints "Bar!" | |
| 51 * } | |
| 52 * b.Foo(); // prints nothing because |a| has already been destroyed | |
| 53 * } | |
| 54 * | |
| 55 * One can imagine much more complex examples, especially when asynchronous | |
| 56 * event processing is involved. | |
| 57 * | |
| 58 * Keep in mind that you should only ever need a class like this when you have | |
| 59 * multiple instances of B, such that it is not possible for A and B to simply | |
| 60 * have pointers to one another. | |
| 61 */ | |
| 62 template <class Type> | |
| 63 class nsTWeakRef { | |
| 64 public: | |
| 65 ~nsTWeakRef() { | |
| 66 if (mRef) | |
| 67 mRef->Release(); | |
| 68 } | |
| 69 | |
| 70 /** | |
| 71 * Construct from an object pointer (may be null). | |
| 72 */ | |
| 73 explicit | |
| 74 nsTWeakRef(Type *obj = nullptr) { | |
| 75 if (obj) { | |
| 76 mRef = new Inner(obj); | |
| 77 } else { | |
| 78 mRef = nullptr; | |
| 79 } | |
| 80 } | |
| 81 | |
| 82 /** | |
| 83 * Construct from another weak reference object. | |
| 84 */ | |
| 85 explicit | |
| 86 nsTWeakRef(const nsTWeakRef<Type> &other) : mRef(other.mRef) { | |
| 87 if (mRef) | |
| 88 mRef->AddRef(); | |
| 89 } | |
| 90 | |
| 91 /** | |
| 92 * Assign from an object pointer. | |
| 93 */ | |
| 94 nsTWeakRef<Type> &operator=(Type *obj) { | |
| 95 if (mRef) | |
| 96 mRef->Release(); | |
| 97 if (obj) { | |
| 98 mRef = new Inner(obj); | |
| 99 } else { | |
| 100 mRef = nullptr; | |
| 101 } | |
| 102 return *this; | |
| 103 } | |
| 104 | |
| 105 /** | |
| 106 * Assign from another weak reference object. | |
| 107 */ | |
| 108 nsTWeakRef<Type> &operator=(const nsTWeakRef<Type> &other) { | |
| 109 if (mRef) | |
| 110 mRef->Release(); | |
| 111 mRef = other.mRef; | |
| 112 if (mRef) | |
| 113 mRef->AddRef(); | |
| 114 return *this; | |
| 115 } | |
| 116 | |
| 117 /** | |
| 118 * Get the referenced object. This method may return null if the reference | |
| 119 * has been cleared or if an out-of-memory error occurred at assignment. | |
| 120 */ | |
| 121 Type *get() const { | |
| 122 return mRef ? mRef->mObj : nullptr; | |
| 123 } | |
| 124 | |
| 125 /** | |
| 126 * Called to "null out" the weak reference. Typically, the object referenced | |
| 127 * by this weak reference calls this method when it is being destroyed. | |
| 128 * @returns The former referenced object. | |
| 129 */ | |
| 130 Type *forget() { | |
| 131 Type *obj; | |
| 132 if (mRef) { | |
| 133 obj = mRef->mObj; | |
| 134 mRef->mObj = nullptr; | |
| 135 mRef->Release(); | |
| 136 mRef = nullptr; | |
| 137 } else { | |
| 138 obj = nullptr; | |
| 139 } | |
| 140 return obj; | |
| 141 } | |
| 142 | |
| 143 /** | |
| 144 * Allow |*this| to be treated as a |Type*| for convenience. | |
| 145 */ | |
| 146 operator Type *() const { | |
| 147 return get(); | |
| 148 } | |
| 149 | |
| 150 /** | |
| 151 * Allow |*this| to be treated as a |Type*| for convenience. Use with | |
| 152 * caution since this method will crash if the referenced object is null. | |
| 153 */ | |
| 154 Type *operator->() const { | |
| 155 NS_ASSERTION(mRef && mRef->mObj, | |
| 156 "You can't dereference a null weak reference with operator->()."); | |
| 157 return get(); | |
| 158 } | |
| 159 | |
| 160 private: | |
| 161 | |
| 162 struct Inner { | |
| 163 int mCnt; | |
| 164 Type *mObj; | |
| 165 | |
| 166 Inner(Type *obj) : mCnt(1), mObj(obj) {} | |
| 167 void AddRef() { ++mCnt; } | |
| 168 void Release() { if (--mCnt == 0) delete this; } | |
| 169 }; | |
| 170 | |
| 171 Inner *mRef; | |
| 172 }; | |
| 173 | |
| 174 #endif // nsTWeakRef_h__ |
