1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
4
5 // Weak pointers are pointers to an object that do not affect its lifetime,
6 // and which may be invalidated (i.e. reset to nullptr) by the object, or its
7 // owner, at any time, most commonly when the object is about to be deleted.
8
9 // Weak pointers are useful when an object needs to be accessed safely by one
10 // or more objects other than its owner, and those callers can cope with the
11 // object vanishing and e.g. tasks posted to it being silently dropped.
12 // Reference-counting such an object would complicate the ownership graph and
13 // make it harder to reason about the object's lifetime.
14
15 // EXAMPLE:
16 //
17 // class Controller {
18 // public:
19 // Controller() : weak_factory_(this) {}
20 // void SpawnWorker() { Worker::StartNew(weak_factory_.GetWeakPtr()); }
21 // void WorkComplete(const Result& result) { ... }
22 // private:
23 // // Member variables should appear before the WeakPtrFactory, to ensure
24 // // that any WeakPtrs to Controller are invalidated before its members
25 // // variable's destructors are executed, rendering them invalid.
26 // WeakPtrFactory<Controller> weak_factory_;
27 // };
28 //
29 // class Worker {
30 // public:
31 // static void StartNew(const WeakPtr<Controller>& controller) {
32 // Worker* worker = new Worker(controller);
33 // // Kick off asynchronous processing...
34 // }
35 // private:
36 // Worker(const WeakPtr<Controller>& controller)
37 // : controller_(controller) {}
38 // void DidCompleteAsynchronousProcessing(const Result& result) {
39 // if (controller_)
40 // controller_->WorkComplete(result);
41 // }
42 // WeakPtr<Controller> controller_;
43 // };
44 //
45 // With this implementation a caller may use SpawnWorker() to dispatch multiple
46 // Workers and subsequently delete the Controller, without waiting for all
47 // Workers to have completed.
48
49 // ------------------------- IMPORTANT: Thread-safety -------------------------
50
51 // Weak pointers may be passed safely between threads, but must always be
52 // dereferenced and invalidated on the same SequencedTaskRunner otherwise
53 // checking the pointer would be racey.
54 //
55 // To ensure correct use, the first time a WeakPtr issued by a WeakPtrFactory
56 // is dereferenced, the factory and its WeakPtrs become bound to the calling
57 // thread or current SequencedWorkerPool token, and cannot be dereferenced or
58 // invalidated on any other task runner. Bound WeakPtrs can still be handed
59 // off to other task runners, e.g. to use to post tasks back to object on the
60 // bound sequence.
61 //
62 // If all WeakPtr objects are destroyed or invalidated then the factory is
63 // unbound from the SequencedTaskRunner/Thread. The WeakPtrFactory may then be
64 // destroyed, or new WeakPtr objects may be used, from a different sequence.
65 //
66 // Thus, at least one WeakPtr object must exist and have been dereferenced on
67 // the correct thread to enforce that other WeakPtr objects will enforce they
68 // are used on the desired thread.
69
70 #ifndef BASE_MEMORY_WEAK_PTR_H_
71 #define BASE_MEMORY_WEAK_PTR_H_
72
73 #include <cstddef>
74 #include <type_traits>
75
76 #include "base/base_export.h"
77 #include "base/logging.h"
78 #include "base/macros.h"
79 #include "base/memory/ref_counted.h"
80 #include "base/sequence_checker.h"
81
82 namespace base {
83
84 template <typename T> class SupportsWeakPtr;
85 template <typename T> class WeakPtr;
86
87 namespace internal {
88 // These classes are part of the WeakPtr implementation.
89 // DO NOT USE THESE CLASSES DIRECTLY YOURSELF.
90
91 class BASE_EXPORT WeakReference {
92 public:
93 // Although Flag is bound to a specific SequencedTaskRunner, it may be
94 // deleted from another via base::WeakPtr::~WeakPtr().
95 class BASE_EXPORT Flag : public RefCountedThreadSafe<Flag> {
96 public:
97 Flag();
98
99 void Invalidate();
100 bool IsValid() const;
101
102 private:
103 friend class base::RefCountedThreadSafe<Flag>;
104
105 ~Flag();
106
107 SequenceChecker sequence_checker_;
108 bool is_valid_;
109 };
110
111 WeakReference();
112 explicit WeakReference(const scoped_refptr<Flag>& flag);
113 ~WeakReference();
114
115 WeakReference(WeakReference&& other);
116 WeakReference(const WeakReference& other);
117 WeakReference& operator=(WeakReference&& other) = default;
118 WeakReference& operator=(const WeakReference& other) = default;
119
120 bool is_valid() const;
121
122 private:
123 scoped_refptr<const Flag> flag_;
124 };
125
126 class BASE_EXPORT WeakReferenceOwner {
127 public:
128 WeakReferenceOwner();
129 ~WeakReferenceOwner();
130
131 WeakReference GetRef() const;
132
HasRefs()133 bool HasRefs() const { return flag_ && !flag_->HasOneRef(); }
134
135 void Invalidate();
136
137 private:
138 mutable scoped_refptr<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 class BASE_EXPORT WeakPtrBase {
146 public:
147 WeakPtrBase();
148 ~WeakPtrBase();
149
150 WeakPtrBase(const WeakPtrBase& other) = default;
151 WeakPtrBase(WeakPtrBase&& other) = default;
152 WeakPtrBase& operator=(const WeakPtrBase& other) = default;
153 WeakPtrBase& operator=(WeakPtrBase&& other) = default;
154
reset()155 void reset() {
156 ref_ = internal::WeakReference();
157 ptr_ = 0;
158 }
159
160 protected:
161 WeakPtrBase(const WeakReference& ref, uintptr_t ptr);
162
163 WeakReference ref_;
164
165 // This pointer is only valid when ref_.is_valid() is true. Otherwise, its
166 // value is undefined (as opposed to nullptr).
167 uintptr_t ptr_;
168 };
169
170 // This class provides a common implementation of common functions that would
171 // otherwise get instantiated separately for each distinct instantiation of
172 // SupportsWeakPtr<>.
173 class SupportsWeakPtrBase {
174 public:
175 // A safe static downcast of a WeakPtr<Base> to WeakPtr<Derived>. This
176 // conversion will only compile if there is exists a Base which inherits
177 // from SupportsWeakPtr<Base>. See base::AsWeakPtr() below for a helper
178 // function that makes calling this easier.
179 //
180 // Precondition: t != nullptr
181 template<typename Derived>
StaticAsWeakPtr(Derived * t)182 static WeakPtr<Derived> StaticAsWeakPtr(Derived* t) {
183 static_assert(
184 std::is_base_of<internal::SupportsWeakPtrBase, Derived>::value,
185 "AsWeakPtr argument must inherit from SupportsWeakPtr");
186 return AsWeakPtrImpl<Derived>(t);
187 }
188
189 private:
190 // This template function uses type inference to find a Base of Derived
191 // which is an instance of SupportsWeakPtr<Base>. We can then safely
192 // static_cast the Base* to a Derived*.
193 template <typename Derived, typename Base>
AsWeakPtrImpl(SupportsWeakPtr<Base> * t)194 static WeakPtr<Derived> AsWeakPtrImpl(SupportsWeakPtr<Base>* t) {
195 WeakPtr<Base> ptr = t->AsWeakPtr();
196 return WeakPtr<Derived>(
197 ptr.ref_, static_cast<Derived*>(reinterpret_cast<Base*>(ptr.ptr_)));
198 }
199 };
200
201 } // namespace internal
202
203 template <typename T> class WeakPtrFactory;
204
205 // The WeakPtr class holds a weak reference to |T*|.
206 //
207 // This class is designed to be used like a normal pointer. You should always
208 // null-test an object of this class before using it or invoking a method that
209 // may result in the underlying object being destroyed.
210 //
211 // EXAMPLE:
212 //
213 // class Foo { ... };
214 // WeakPtr<Foo> foo;
215 // if (foo)
216 // foo->method();
217 //
218 template <typename T>
219 class WeakPtr : public internal::WeakPtrBase {
220 public:
221 WeakPtr() = default;
222
WeakPtr(std::nullptr_t)223 WeakPtr(std::nullptr_t) {}
224
225 // Allow conversion from U to T provided U "is a" T. Note that this
226 // is separate from the (implicit) copy and move constructors.
227 template <typename U>
WeakPtr(const WeakPtr<U> & other)228 WeakPtr(const WeakPtr<U>& other) : WeakPtrBase(other) {
229 // Need to cast from U* to T* to do pointer adjustment in case of multiple
230 // inheritance. This also enforces the "U is a T" rule.
231 T* t = reinterpret_cast<U*>(other.ptr_);
232 ptr_ = reinterpret_cast<uintptr_t>(t);
233 }
234 template <typename U>
WeakPtr(WeakPtr<U> && other)235 WeakPtr(WeakPtr<U>&& other) : WeakPtrBase(std::move(other)) {
236 // Need to cast from U* to T* to do pointer adjustment in case of multiple
237 // inheritance. This also enforces the "U is a T" rule.
238 T* t = reinterpret_cast<U*>(other.ptr_);
239 ptr_ = reinterpret_cast<uintptr_t>(t);
240 }
241
get()242 T* get() const {
243 return ref_.is_valid() ? reinterpret_cast<T*>(ptr_) : nullptr;
244 }
245
246 T& operator*() const {
247 DCHECK(get() != nullptr);
248 return *get();
249 }
250 T* operator->() const {
251 DCHECK(get() != nullptr);
252 return get();
253 }
254
255 // Allow conditionals to test validity, e.g. if (weak_ptr) {...};
256 explicit operator bool() const { return get() != nullptr; }
257
258 private:
259 friend class internal::SupportsWeakPtrBase;
260 template <typename U> friend class WeakPtr;
261 friend class SupportsWeakPtr<T>;
262 friend class WeakPtrFactory<T>;
263
WeakPtr(const internal::WeakReference & ref,T * ptr)264 WeakPtr(const internal::WeakReference& ref, T* ptr)
265 : WeakPtrBase(ref, reinterpret_cast<uintptr_t>(ptr)) {}
266 };
267
268 // Allow callers to compare WeakPtrs against nullptr to test validity.
269 template <class T>
270 bool operator!=(const WeakPtr<T>& weak_ptr, std::nullptr_t) {
271 return !(weak_ptr == nullptr);
272 }
273 template <class T>
274 bool operator!=(std::nullptr_t, const WeakPtr<T>& weak_ptr) {
275 return weak_ptr != nullptr;
276 }
277 template <class T>
278 bool operator==(const WeakPtr<T>& weak_ptr, std::nullptr_t) {
279 return weak_ptr.get() == nullptr;
280 }
281 template <class T>
282 bool operator==(std::nullptr_t, const WeakPtr<T>& weak_ptr) {
283 return weak_ptr == nullptr;
284 }
285
286 namespace internal {
287 class BASE_EXPORT WeakPtrFactoryBase {
288 protected:
289 WeakPtrFactoryBase(uintptr_t ptr);
290 ~WeakPtrFactoryBase();
291 internal::WeakReferenceOwner weak_reference_owner_;
292 uintptr_t ptr_;
293 };
294 } // namespace internal
295
296 // A class may be composed of a WeakPtrFactory and thereby
297 // control how it exposes weak pointers to itself. This is helpful if you only
298 // need weak pointers within the implementation of a class. This class is also
299 // useful when working with primitive types. For example, you could have a
300 // WeakPtrFactory<bool> that is used to pass around a weak reference to a bool.
301 template <class T>
302 class WeakPtrFactory : public internal::WeakPtrFactoryBase {
303 public:
WeakPtrFactory(T * ptr)304 explicit WeakPtrFactory(T* ptr)
305 : WeakPtrFactoryBase(reinterpret_cast<uintptr_t>(ptr)) {}
306
307 ~WeakPtrFactory() = default;
308
GetWeakPtr()309 WeakPtr<T> GetWeakPtr() {
310 return WeakPtr<T>(weak_reference_owner_.GetRef(),
311 reinterpret_cast<T*>(ptr_));
312 }
313
314 // Call this method to invalidate all existing weak pointers.
InvalidateWeakPtrs()315 void InvalidateWeakPtrs() {
316 DCHECK(ptr_);
317 weak_reference_owner_.Invalidate();
318 }
319
320 // Call this method to determine if any weak pointers exist.
HasWeakPtrs()321 bool HasWeakPtrs() const {
322 DCHECK(ptr_);
323 return weak_reference_owner_.HasRefs();
324 }
325
326 private:
327 DISALLOW_IMPLICIT_CONSTRUCTORS(WeakPtrFactory);
328 };
329
330 // A class may extend from SupportsWeakPtr to let others take weak pointers to
331 // it. This avoids the class itself implementing boilerplate to dispense weak
332 // pointers. However, since SupportsWeakPtr's destructor won't invalidate
333 // weak pointers to the class until after the derived class' members have been
334 // destroyed, its use can lead to subtle use-after-destroy issues.
335 template <class T>
336 class SupportsWeakPtr : public internal::SupportsWeakPtrBase {
337 public:
338 SupportsWeakPtr() = default;
339
AsWeakPtr()340 WeakPtr<T> AsWeakPtr() {
341 return WeakPtr<T>(weak_reference_owner_.GetRef(), static_cast<T*>(this));
342 }
343
344 protected:
345 ~SupportsWeakPtr() = default;
346
347 private:
348 internal::WeakReferenceOwner weak_reference_owner_;
349 DISALLOW_COPY_AND_ASSIGN(SupportsWeakPtr);
350 };
351
352 // Helper function that uses type deduction to safely return a WeakPtr<Derived>
353 // when Derived doesn't directly extend SupportsWeakPtr<Derived>, instead it
354 // extends a Base that extends SupportsWeakPtr<Base>.
355 //
356 // EXAMPLE:
357 // class Base : public base::SupportsWeakPtr<Producer> {};
358 // class Derived : public Base {};
359 //
360 // Derived derived;
361 // base::WeakPtr<Derived> ptr = base::AsWeakPtr(&derived);
362 //
363 // Note that the following doesn't work (invalid type conversion) since
364 // Derived::AsWeakPtr() is WeakPtr<Base> SupportsWeakPtr<Base>::AsWeakPtr(),
365 // and there's no way to safely cast WeakPtr<Base> to WeakPtr<Derived> at
366 // the caller.
367 //
368 // base::WeakPtr<Derived> ptr = derived.AsWeakPtr(); // Fails.
369
370 template <typename Derived>
AsWeakPtr(Derived * t)371 WeakPtr<Derived> AsWeakPtr(Derived* t) {
372 return internal::SupportsWeakPtrBase::StaticAsWeakPtr<Derived>(t);
373 }
374
375 } // namespace base
376
377 #endif // BASE_MEMORY_WEAK_PTR_H_
378