Line data Source code
1 : /*
2 : * Copyright 2016 The WebRTC Project Authors. All rights reserved.
3 : *
4 : * Use of this source code is governed by a BSD-style license
5 : * that can be found in the LICENSE file in the root of the source
6 : * tree. An additional intellectual property rights grant can be found
7 : * in the file PATENTS. All contributing project authors may
8 : * be found in the AUTHORS file in the root of the source tree.
9 : */
10 :
11 : #ifndef WEBRTC_BASE_WEAK_PTR_H_
12 : #define WEBRTC_BASE_WEAK_PTR_H_
13 :
14 : #include <memory>
15 :
16 : #include <utility>
17 :
18 : #include "webrtc/base/refcount.h"
19 : #include "webrtc/base/scoped_ref_ptr.h"
20 : #include "webrtc/base/sequenced_task_checker.h"
21 :
22 : // The implementation is borrowed from chromium except that it does not
23 : // implement SupportsWeakPtr.
24 :
25 : // Weak pointers are pointers to an object that do not affect its lifetime,
26 : // and which may be invalidated (i.e. reset to nullptr) by the object, or its
27 : // owner, at any time, most commonly when the object is about to be deleted.
28 :
29 : // Weak pointers are useful when an object needs to be accessed safely by one
30 : // or more objects other than its owner, and those callers can cope with the
31 : // object vanishing and e.g. tasks posted to it being silently dropped.
32 : // Reference-counting such an object would complicate the ownership graph and
33 : // make it harder to reason about the object's lifetime.
34 :
35 : // EXAMPLE:
36 : //
37 : // class Controller {
38 : // public:
39 : // Controller() : weak_factory_(this) {}
40 : // void SpawnWorker() { Worker::StartNew(weak_factory_.GetWeakPtr()); }
41 : // void WorkComplete(const Result& result) { ... }
42 : // private:
43 : // // Member variables should appear before the WeakPtrFactory, to ensure
44 : // // that any WeakPtrs to Controller are invalidated before its members
45 : // // variable's destructors are executed, rendering them invalid.
46 : // WeakPtrFactory<Controller> weak_factory_;
47 : // };
48 : //
49 : // class Worker {
50 : // public:
51 : // static void StartNew(const WeakPtr<Controller>& controller) {
52 : // Worker* worker = new Worker(controller);
53 : // // Kick off asynchronous processing...
54 : // }
55 : // private:
56 : // Worker(const WeakPtr<Controller>& controller)
57 : // : controller_(controller) {}
58 : // void DidCompleteAsynchronousProcessing(const Result& result) {
59 : // if (controller_)
60 : // controller_->WorkComplete(result);
61 : // }
62 : // WeakPtr<Controller> controller_;
63 : // };
64 : //
65 : // With this implementation a caller may use SpawnWorker() to dispatch multiple
66 : // Workers and subsequently delete the Controller, without waiting for all
67 : // Workers to have completed.
68 :
69 : // ------------------------- IMPORTANT: Thread-safety -------------------------
70 :
71 : // Weak pointers may be passed safely between threads, but must always be
72 : // dereferenced and invalidated on the same TaskQueue or thread, otherwise
73 : // checking the pointer would be racey.
74 : //
75 : // To ensure correct use, the first time a WeakPtr issued by a WeakPtrFactory
76 : // is dereferenced, the factory and its WeakPtrs become bound to the calling
77 : // TaskQueue/thread, and cannot be dereferenced or
78 : // invalidated on any other TaskQueue/thread. Bound WeakPtrs can still be handed
79 : // off to other TaskQueues, e.g. to use to post tasks back to object on the
80 : // bound sequence.
81 : //
82 : // Thus, at least one WeakPtr object must exist and have been dereferenced on
83 : // the correct thread to enforce that other WeakPtr objects will enforce they
84 : // are used on the desired thread.
85 :
86 : namespace rtc {
87 :
88 : namespace internal {
89 :
90 0 : class WeakReference {
91 : public:
92 : // Although Flag is bound to a specific sequence, it may be
93 : // deleted from another via base::WeakPtr::~WeakPtr().
94 : class Flag : public RefCountInterface {
95 : public:
96 : Flag();
97 :
98 : void Invalidate();
99 : bool IsValid() const;
100 :
101 : private:
102 : friend class RefCountedObject<Flag>;
103 :
104 : ~Flag() override;
105 :
106 : SequencedTaskChecker checker_;
107 : bool is_valid_;
108 : };
109 :
110 : WeakReference();
111 : explicit WeakReference(const Flag* flag);
112 : ~WeakReference();
113 :
114 : WeakReference(WeakReference&& other);
115 : WeakReference(const WeakReference& other);
116 : WeakReference& operator=(WeakReference&& other) = default;
117 : WeakReference& operator=(const WeakReference& other) = default;
118 :
119 : bool is_valid() const;
120 :
121 : private:
122 : scoped_refptr<const Flag> flag_;
123 : };
124 :
125 : class WeakReferenceOwner {
126 : public:
127 : WeakReferenceOwner();
128 : ~WeakReferenceOwner();
129 :
130 : WeakReference GetRef() const;
131 :
132 0 : bool HasRefs() const { return flag_.get() && !flag_->HasOneRef(); }
133 :
134 : void Invalidate();
135 :
136 : private:
137 : SequencedTaskChecker checker_;
138 : mutable scoped_refptr<RefCountedObject<WeakReference::Flag>> flag_;
139 : };
140 :
141 : // This class simplifies the implementation of WeakPtr's type conversion
142 : // constructor by avoiding the need for a public accessor for ref_. A
143 : // WeakPtr<T> cannot access the private members of WeakPtr<U>, so this
144 : // base class gives us a way to access ref_ in a protected fashion.
145 0 : class WeakPtrBase {
146 : public:
147 : WeakPtrBase();
148 : ~WeakPtrBase();
149 :
150 0 : WeakPtrBase(const WeakPtrBase& other) = default;
151 : WeakPtrBase(WeakPtrBase&& other) = default;
152 : WeakPtrBase& operator=(const WeakPtrBase& other) = default;
153 : WeakPtrBase& operator=(WeakPtrBase&& other) = default;
154 :
155 : protected:
156 : explicit WeakPtrBase(const WeakReference& ref);
157 :
158 : WeakReference ref_;
159 : };
160 :
161 : } // namespace internal
162 :
163 : template <typename T>
164 : class WeakPtrFactory;
165 :
166 : template <typename T>
167 0 : class WeakPtr : public internal::WeakPtrBase {
168 : public:
169 0 : WeakPtr() : ptr_(nullptr) {}
170 :
171 : // Allow conversion from U to T provided U "is a" T. Note that this
172 : // is separate from the (implicit) copy and move constructors.
173 : template <typename U>
174 : WeakPtr(const WeakPtr<U>& other)
175 : : internal::WeakPtrBase(other), ptr_(other.ptr_) {}
176 : template <typename U>
177 : WeakPtr(WeakPtr<U>&& other)
178 : : internal::WeakPtrBase(std::move(other)), ptr_(other.ptr_) {}
179 :
180 0 : T* get() const { return ref_.is_valid() ? ptr_ : nullptr; }
181 :
182 : T& operator*() const {
183 : RTC_DCHECK(get() != nullptr);
184 : return *get();
185 : }
186 0 : T* operator->() const {
187 0 : RTC_DCHECK(get() != nullptr);
188 0 : return get();
189 : }
190 :
191 0 : void reset() {
192 0 : ref_ = internal::WeakReference();
193 0 : ptr_ = nullptr;
194 0 : }
195 :
196 : // Allow conditionals to test validity, e.g. if (weak_ptr) {...};
197 0 : explicit operator bool() const { return get() != nullptr; }
198 :
199 : private:
200 : template <typename U>
201 : friend class WeakPtr;
202 : friend class WeakPtrFactory<T>;
203 :
204 0 : WeakPtr(const internal::WeakReference& ref, T* ptr)
205 0 : : internal::WeakPtrBase(ref), ptr_(ptr) {}
206 :
207 : // This pointer is only valid when ref_.is_valid() is true. Otherwise, its
208 : // value is undefined (as opposed to nullptr).
209 : T* ptr_;
210 : };
211 :
212 : // Allow callers to compare WeakPtrs against nullptr to test validity.
213 : template <class T>
214 : bool operator!=(const WeakPtr<T>& weak_ptr, std::nullptr_t) {
215 : return !(weak_ptr == nullptr);
216 : }
217 : template <class T>
218 : bool operator!=(std::nullptr_t, const WeakPtr<T>& weak_ptr) {
219 : return weak_ptr != nullptr;
220 : }
221 : template <class T>
222 : bool operator==(const WeakPtr<T>& weak_ptr, std::nullptr_t) {
223 : return weak_ptr.get() == nullptr;
224 : }
225 : template <class T>
226 : bool operator==(std::nullptr_t, const WeakPtr<T>& weak_ptr) {
227 : return weak_ptr == nullptr;
228 : }
229 :
230 : // A class may be composed of a WeakPtrFactory and thereby
231 : // control how it exposes weak pointers to itself. This is helpful if you only
232 : // need weak pointers within the implementation of a class. This class is also
233 : // useful when working with primitive types. For example, you could have a
234 : // WeakPtrFactory<bool> that is used to pass around a weak reference to a bool.
235 :
236 : // Note that GetWeakPtr must be called on one and only one TaskQueue or thread
237 : // and the WeakPtr must only be dereferenced and invalidated on that same
238 : // TaskQueue/thread. A WeakPtr instance can be copied and posted to other
239 : // sequences though as long as it is not dereferenced (WeakPtr<T>::get()).
240 : template <class T>
241 : class WeakPtrFactory {
242 : public:
243 0 : explicit WeakPtrFactory(T* ptr) : ptr_(ptr) {}
244 :
245 0 : ~WeakPtrFactory() { ptr_ = nullptr; }
246 :
247 0 : WeakPtr<T> GetWeakPtr() {
248 0 : RTC_DCHECK(ptr_);
249 0 : return WeakPtr<T>(weak_reference_owner_.GetRef(), ptr_);
250 : }
251 :
252 : // Call this method to invalidate all existing weak pointers.
253 : void InvalidateWeakPtrs() {
254 : RTC_DCHECK(ptr_);
255 : weak_reference_owner_.Invalidate();
256 : }
257 :
258 : // Call this method to determine if any weak pointers exist.
259 : bool HasWeakPtrs() const {
260 : RTC_DCHECK(ptr_);
261 : return weak_reference_owner_.HasRefs();
262 : }
263 :
264 : private:
265 : internal::WeakReferenceOwner weak_reference_owner_;
266 : T* ptr_;
267 : RTC_DISALLOW_IMPLICIT_CONSTRUCTORS(WeakPtrFactory);
268 : };
269 :
270 : } // namespace rtc
271 :
272 : #endif // WEBRTC_BASE_WEAK_PTR_H_
|
