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 // void SpawnWorker() { Worker::StartNew(weak_factory_.GetWeakPtr()); }
20 // void WorkComplete(const Result& result) { ... }
21 // private:
22 // // Member variables should appear before the WeakPtrFactory, to ensure
23 // // that any WeakPtrs to Controller are invalidated before its members
24 // // variable's destructors are executed, rendering them invalid.
25 // WeakPtrFactory<Controller> weak_factory_{this};
26 // };
27 //
28 // class Worker {
29 // public:
30 // static void StartNew(const WeakPtr<Controller>& controller) {
31 // Worker* worker = new Worker(controller);
32 // // Kick off asynchronous processing...
33 // }
34 // private:
35 // Worker(const WeakPtr<Controller>& controller)
36 // : controller_(controller) {}
37 // void DidCompleteAsynchronousProcessing(const Result& result) {
38 // if (controller_)
39 // controller_->WorkComplete(result);
40 // }
41 // WeakPtr<Controller> controller_;
42 // };
43 //
44 // With this implementation a caller may use SpawnWorker() to dispatch multiple
45 // Workers and subsequently delete the Controller, without waiting for all
46 // Workers to have completed.
47
48 // ------------------------- IMPORTANT: Thread-safety -------------------------
49
50 // Weak pointers may be passed safely between sequences, but must always be
51 // dereferenced and invalidated on the same SequencedTaskRunner otherwise
52 // checking the pointer would be racey.
53 //
54 // To ensure correct use, the first time a WeakPtr issued by a WeakPtrFactory
55 // is dereferenced, the factory and its WeakPtrs become bound to the calling
56 // sequence or current SequencedWorkerPool token, and cannot be dereferenced or
57 // invalidated on any other task runner. Bound WeakPtrs can still be handed
58 // off to other task runners, e.g. to use to post tasks back to object on the
59 // bound sequence.
60 //
61 // If all WeakPtr objects are destroyed or invalidated then the factory is
62 // unbound from the SequencedTaskRunner/Thread. The WeakPtrFactory may then be
63 // destroyed, or new WeakPtr objects may be used, from a different sequence.
64 //
65 // Thus, at least one WeakPtr object must exist and have been dereferenced on
66 // the correct sequence to enforce that other WeakPtr objects will enforce they
67 // are used on the desired sequence.
68
69 #ifndef BASE_MEMORY_WEAK_PTR_H_
70 #define BASE_MEMORY_WEAK_PTR_H_
71
72 #include <cstddef>
73 #include <type_traits>
74
75 #include "base/base_export.h"
76 #include "base/logging.h"
77 #include "base/macros.h"
78 #include "base/memory/ref_counted.h"
79 #include "base/sequence_checker.h"
80 #include "base/synchronization/atomic_flag.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 bool MaybeValid() const;
103
104 void DetachFromSequence();
105
106 private:
107 friend class base::RefCountedThreadSafe<Flag>;
108
109 ~Flag();
110
111 SEQUENCE_CHECKER(sequence_checker_);
112 AtomicFlag invalidated_;
113 };
114
115 WeakReference();
116 explicit WeakReference(const scoped_refptr<Flag>& flag);
117 ~WeakReference();
118
119 WeakReference(WeakReference&& other) noexcept;
120 WeakReference(const WeakReference& other);
121 WeakReference& operator=(WeakReference&& other) noexcept = default;
122 WeakReference& operator=(const WeakReference& other) = default;
123
124 bool IsValid() const;
125 bool MaybeValid() const;
126
127 private:
128 scoped_refptr<const Flag> flag_;
129 };
130
131 class BASE_EXPORT WeakReferenceOwner {
132 public:
133 WeakReferenceOwner();
134 ~WeakReferenceOwner();
135 WeakReferenceOwner(WeakReferenceOwner&& other) noexcept = default;
136 WeakReferenceOwner(const WeakReferenceOwner& other) = default;
137 WeakReferenceOwner& operator=(WeakReferenceOwner&& other) noexcept = default;
138 WeakReferenceOwner& operator=(const WeakReferenceOwner& other) = default;
139
140 WeakReference GetRef() const;
141
HasRefs()142 bool HasRefs() const { return !flag_->HasOneRef(); }
143
144 void Invalidate();
145
146 private:
147 scoped_refptr<WeakReference::Flag> flag_;
148 };
149
150 // This class simplifies the implementation of WeakPtr's type conversion
151 // constructor by avoiding the need for a public accessor for ref_. A
152 // WeakPtr<T> cannot access the private members of WeakPtr<U>, so this
153 // base class gives us a way to access ref_ in a protected fashion.
154 class BASE_EXPORT WeakPtrBase {
155 public:
156 WeakPtrBase();
157 ~WeakPtrBase();
158
159 WeakPtrBase(const WeakPtrBase& other) = default;
160 WeakPtrBase(WeakPtrBase&& other) noexcept = default;
161 WeakPtrBase& operator=(const WeakPtrBase& other) = default;
162 WeakPtrBase& operator=(WeakPtrBase&& other) noexcept = default;
163
reset()164 void reset() {
165 ref_ = internal::WeakReference();
166 ptr_ = 0;
167 }
168
169 protected:
170 WeakPtrBase(const WeakReference& ref, uintptr_t ptr);
171
172 WeakReference ref_;
173
174 // This pointer is only valid when ref_.is_valid() is true. Otherwise, its
175 // value is undefined (as opposed to nullptr).
176 uintptr_t ptr_;
177 };
178
179 // This class provides a common implementation of common functions that would
180 // otherwise get instantiated separately for each distinct instantiation of
181 // SupportsWeakPtr<>.
182 class SupportsWeakPtrBase {
183 public:
184 // A safe static downcast of a WeakPtr<Base> to WeakPtr<Derived>. This
185 // conversion will only compile if there is exists a Base which inherits
186 // from SupportsWeakPtr<Base>. See base::AsWeakPtr() below for a helper
187 // function that makes calling this easier.
188 //
189 // Precondition: t != nullptr
190 template<typename Derived>
StaticAsWeakPtr(Derived * t)191 static WeakPtr<Derived> StaticAsWeakPtr(Derived* t) {
192 static_assert(
193 std::is_base_of<internal::SupportsWeakPtrBase, Derived>::value,
194 "AsWeakPtr argument must inherit from SupportsWeakPtr");
195 return AsWeakPtrImpl<Derived>(t);
196 }
197
198 private:
199 // This template function uses type inference to find a Base of Derived
200 // which is an instance of SupportsWeakPtr<Base>. We can then safely
201 // static_cast the Base* to a Derived*.
202 template <typename Derived, typename Base>
AsWeakPtrImpl(SupportsWeakPtr<Base> * t)203 static WeakPtr<Derived> AsWeakPtrImpl(SupportsWeakPtr<Base>* t) {
204 WeakPtr<Base> ptr = t->AsWeakPtr();
205 return WeakPtr<Derived>(
206 ptr.ref_, static_cast<Derived*>(reinterpret_cast<Base*>(ptr.ptr_)));
207 }
208 };
209
210 } // namespace internal
211
212 template <typename T> class WeakPtrFactory;
213
214 // The WeakPtr class holds a weak reference to |T*|.
215 //
216 // This class is designed to be used like a normal pointer. You should always
217 // null-test an object of this class before using it or invoking a method that
218 // may result in the underlying object being destroyed.
219 //
220 // EXAMPLE:
221 //
222 // class Foo { ... };
223 // WeakPtr<Foo> foo;
224 // if (foo)
225 // foo->method();
226 //
227 template <typename T>
228 class WeakPtr : public internal::WeakPtrBase {
229 public:
230 WeakPtr() = default;
WeakPtr(std::nullptr_t)231 WeakPtr(std::nullptr_t) {}
232
233 // Allow conversion from U to T provided U "is a" T. Note that this
234 // is separate from the (implicit) copy and move constructors.
235 template <typename U>
WeakPtr(const WeakPtr<U> & other)236 WeakPtr(const WeakPtr<U>& other) : WeakPtrBase(other) {
237 // Need to cast from U* to T* to do pointer adjustment in case of multiple
238 // inheritance. This also enforces the "U is a T" rule.
239 T* t = reinterpret_cast<U*>(other.ptr_);
240 ptr_ = reinterpret_cast<uintptr_t>(t);
241 }
242 template <typename U>
WeakPtr(WeakPtr<U> && other)243 WeakPtr(WeakPtr<U>&& other) noexcept : WeakPtrBase(std::move(other)) {
244 // Need to cast from U* to T* to do pointer adjustment in case of multiple
245 // inheritance. This also enforces the "U is a T" rule.
246 T* t = reinterpret_cast<U*>(other.ptr_);
247 ptr_ = reinterpret_cast<uintptr_t>(t);
248 }
249
get()250 T* get() const {
251 return ref_.IsValid() ? reinterpret_cast<T*>(ptr_) : nullptr;
252 }
253
254 T& operator*() const {
255 DCHECK(get() != nullptr);
256 return *get();
257 }
258 T* operator->() const {
259 DCHECK(get() != nullptr);
260 return get();
261 }
262
263 // Allow conditionals to test validity, e.g. if (weak_ptr) {...};
264 explicit operator bool() const { return get() != nullptr; }
265
266 // Returns false if the WeakPtr is confirmed to be invalid. This call is safe
267 // to make from any thread, e.g. to optimize away unnecessary work, but
268 // operator bool() must always be called, on the correct sequence, before
269 // actually using the pointer.
270 //
271 // Warning: as with any object, this call is only thread-safe if the WeakPtr
272 // instance isn't being re-assigned or reset() racily with this call.
MaybeValid()273 bool MaybeValid() const { return ref_.MaybeValid(); }
274
275 // Returns whether the object |this| points to has been invalidated. This can
276 // be used to distinguish a WeakPtr to a destroyed object from one that has
277 // been explicitly set to null.
WasInvalidated()278 bool WasInvalidated() const { return ptr_ && !ref_.IsValid(); }
279
280 private:
281 friend class internal::SupportsWeakPtrBase;
282 template <typename U> friend class WeakPtr;
283 friend class SupportsWeakPtr<T>;
284 friend class WeakPtrFactory<T>;
285
WeakPtr(const internal::WeakReference & ref,T * ptr)286 WeakPtr(const internal::WeakReference& ref, T* ptr)
287 : WeakPtrBase(ref, reinterpret_cast<uintptr_t>(ptr)) {}
288 };
289
290 // Allow callers to compare WeakPtrs against nullptr to test validity.
291 template <class T>
292 bool operator!=(const WeakPtr<T>& weak_ptr, std::nullptr_t) {
293 return !(weak_ptr == nullptr);
294 }
295 template <class T>
296 bool operator!=(std::nullptr_t, const WeakPtr<T>& weak_ptr) {
297 return weak_ptr != nullptr;
298 }
299 template <class T>
300 bool operator==(const WeakPtr<T>& weak_ptr, std::nullptr_t) {
301 return weak_ptr.get() == nullptr;
302 }
303 template <class T>
304 bool operator==(std::nullptr_t, const WeakPtr<T>& weak_ptr) {
305 return weak_ptr == nullptr;
306 }
307
308 namespace internal {
309 class BASE_EXPORT WeakPtrFactoryBase {
310 protected:
311 WeakPtrFactoryBase(uintptr_t ptr);
312 ~WeakPtrFactoryBase();
313 internal::WeakReferenceOwner weak_reference_owner_;
314 uintptr_t ptr_;
315 };
316 } // namespace internal
317
318 // A class may be composed of a WeakPtrFactory and thereby
319 // control how it exposes weak pointers to itself. This is helpful if you only
320 // need weak pointers within the implementation of a class. This class is also
321 // useful when working with primitive types. For example, you could have a
322 // WeakPtrFactory<bool> that is used to pass around a weak reference to a bool.
323 template <class T>
324 class WeakPtrFactory : public internal::WeakPtrFactoryBase {
325 public:
WeakPtrFactory(T * ptr)326 explicit WeakPtrFactory(T* ptr)
327 : WeakPtrFactoryBase(reinterpret_cast<uintptr_t>(ptr)) {}
328
329 ~WeakPtrFactory() = default;
330 WeakPtrFactory(WeakPtrFactory&&) noexcept = default;
331 WeakPtrFactory& operator=(WeakPtrFactory&&) noexcept = default;
332
GetWeakPtr()333 WeakPtr<T> GetWeakPtr() {
334 return WeakPtr<T>(weak_reference_owner_.GetRef(),
335 reinterpret_cast<T*>(ptr_));
336 }
337
338 // Call this method to invalidate all existing weak pointers.
InvalidateWeakPtrs()339 void InvalidateWeakPtrs() {
340 DCHECK(ptr_);
341 weak_reference_owner_.Invalidate();
342 }
343
344 // Call this method to determine if any weak pointers exist.
HasWeakPtrs()345 bool HasWeakPtrs() const {
346 DCHECK(ptr_);
347 return weak_reference_owner_.HasRefs();
348 }
349
350 private:
351 DISALLOW_IMPLICIT_CONSTRUCTORS(WeakPtrFactory);
352 };
353
354 // A class may extend from SupportsWeakPtr to let others take weak pointers to
355 // it. This avoids the class itself implementing boilerplate to dispense weak
356 // pointers. However, since SupportsWeakPtr's destructor won't invalidate
357 // weak pointers to the class until after the derived class' members have been
358 // destroyed, its use can lead to subtle use-after-destroy issues.
359 template <class T>
360 class SupportsWeakPtr : public internal::SupportsWeakPtrBase {
361 public:
362 SupportsWeakPtr() = default;
363
AsWeakPtr()364 WeakPtr<T> AsWeakPtr() {
365 return WeakPtr<T>(weak_reference_owner_.GetRef(), static_cast<T*>(this));
366 }
367
368 protected:
369 ~SupportsWeakPtr() = default;
370
371 private:
372 internal::WeakReferenceOwner weak_reference_owner_;
373 DISALLOW_COPY_AND_ASSIGN(SupportsWeakPtr);
374 };
375
376 // Helper function that uses type deduction to safely return a WeakPtr<Derived>
377 // when Derived doesn't directly extend SupportsWeakPtr<Derived>, instead it
378 // extends a Base that extends SupportsWeakPtr<Base>.
379 //
380 // EXAMPLE:
381 // class Base : public base::SupportsWeakPtr<Producer> {};
382 // class Derived : public Base {};
383 //
384 // Derived derived;
385 // base::WeakPtr<Derived> ptr = base::AsWeakPtr(&derived);
386 //
387 // Note that the following doesn't work (invalid type conversion) since
388 // Derived::AsWeakPtr() is WeakPtr<Base> SupportsWeakPtr<Base>::AsWeakPtr(),
389 // and there's no way to safely cast WeakPtr<Base> to WeakPtr<Derived> at
390 // the caller.
391 //
392 // base::WeakPtr<Derived> ptr = derived.AsWeakPtr(); // Fails.
393
394 template <typename Derived>
AsWeakPtr(Derived * t)395 WeakPtr<Derived> AsWeakPtr(Derived* t) {
396 return internal::SupportsWeakPtrBase::StaticAsWeakPtr<Derived>(t);
397 }
398
399 } // namespace base
400
401 #endif // BASE_MEMORY_WEAK_PTR_H_
402