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/check.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
136 WeakReference GetRef() const;
137
HasRefs()138 bool HasRefs() const { return !flag_->HasOneRef(); }
139
140 void Invalidate();
141
142 private:
143 scoped_refptr<WeakReference::Flag> flag_;
144 };
145
146 // This class simplifies the implementation of WeakPtr's type conversion
147 // constructor by avoiding the need for a public accessor for ref_. A
148 // WeakPtr<T> cannot access the private members of WeakPtr<U>, so this
149 // base class gives us a way to access ref_ in a protected fashion.
150 class BASE_EXPORT WeakPtrBase {
151 public:
152 WeakPtrBase();
153 ~WeakPtrBase();
154
155 WeakPtrBase(const WeakPtrBase& other) = default;
156 WeakPtrBase(WeakPtrBase&& other) noexcept = default;
157 WeakPtrBase& operator=(const WeakPtrBase& other) = default;
158 WeakPtrBase& operator=(WeakPtrBase&& other) noexcept = default;
159
reset()160 void reset() {
161 ref_ = internal::WeakReference();
162 ptr_ = 0;
163 }
164
165 protected:
166 WeakPtrBase(const WeakReference& ref, uintptr_t ptr);
167
168 WeakReference ref_;
169
170 // This pointer is only valid when ref_.is_valid() is true. Otherwise, its
171 // value is undefined (as opposed to nullptr).
172 uintptr_t ptr_;
173 };
174
175 // This class provides a common implementation of common functions that would
176 // otherwise get instantiated separately for each distinct instantiation of
177 // SupportsWeakPtr<>.
178 class SupportsWeakPtrBase {
179 public:
180 // A safe static downcast of a WeakPtr<Base> to WeakPtr<Derived>. This
181 // conversion will only compile if there is exists a Base which inherits
182 // from SupportsWeakPtr<Base>. See base::AsWeakPtr() below for a helper
183 // function that makes calling this easier.
184 //
185 // Precondition: t != nullptr
186 template<typename Derived>
StaticAsWeakPtr(Derived * t)187 static WeakPtr<Derived> StaticAsWeakPtr(Derived* t) {
188 static_assert(
189 std::is_base_of<internal::SupportsWeakPtrBase, Derived>::value,
190 "AsWeakPtr argument must inherit from SupportsWeakPtr");
191 return AsWeakPtrImpl<Derived>(t);
192 }
193
194 private:
195 // This template function uses type inference to find a Base of Derived
196 // which is an instance of SupportsWeakPtr<Base>. We can then safely
197 // static_cast the Base* to a Derived*.
198 template <typename Derived, typename Base>
AsWeakPtrImpl(SupportsWeakPtr<Base> * t)199 static WeakPtr<Derived> AsWeakPtrImpl(SupportsWeakPtr<Base>* t) {
200 WeakPtr<Base> ptr = t->AsWeakPtr();
201 return WeakPtr<Derived>(
202 ptr.ref_, static_cast<Derived*>(reinterpret_cast<Base*>(ptr.ptr_)));
203 }
204 };
205
206 } // namespace internal
207
208 template <typename T> class WeakPtrFactory;
209
210 // The WeakPtr class holds a weak reference to |T*|.
211 //
212 // This class is designed to be used like a normal pointer. You should always
213 // null-test an object of this class before using it or invoking a method that
214 // may result in the underlying object being destroyed.
215 //
216 // EXAMPLE:
217 //
218 // class Foo { ... };
219 // WeakPtr<Foo> foo;
220 // if (foo)
221 // foo->method();
222 //
223 template <typename T>
224 class WeakPtr : public internal::WeakPtrBase {
225 public:
226 WeakPtr() = default;
WeakPtr(std::nullptr_t)227 WeakPtr(std::nullptr_t) {}
228
229 // Allow conversion from U to T provided U "is a" T. Note that this
230 // is separate from the (implicit) copy and move constructors.
231 template <typename U>
WeakPtr(const WeakPtr<U> & other)232 WeakPtr(const WeakPtr<U>& other) : WeakPtrBase(other) {
233 // Need to cast from U* to T* to do pointer adjustment in case of multiple
234 // inheritance. This also enforces the "U is a T" rule.
235 T* t = reinterpret_cast<U*>(other.ptr_);
236 ptr_ = reinterpret_cast<uintptr_t>(t);
237 }
238 template <typename U>
WeakPtr(WeakPtr<U> && other)239 WeakPtr(WeakPtr<U>&& other) noexcept : WeakPtrBase(std::move(other)) {
240 // Need to cast from U* to T* to do pointer adjustment in case of multiple
241 // inheritance. This also enforces the "U is a T" rule.
242 T* t = reinterpret_cast<U*>(other.ptr_);
243 ptr_ = reinterpret_cast<uintptr_t>(t);
244 }
245
get()246 T* get() const {
247 return ref_.IsValid() ? reinterpret_cast<T*>(ptr_) : nullptr;
248 }
249
250 T& operator*() const {
251 CHECK(ref_.IsValid());
252 return *get();
253 }
254 T* operator->() const {
255 CHECK(ref_.IsValid());
256 return get();
257 }
258
259 // Allow conditionals to test validity, e.g. if (weak_ptr) {...};
260 explicit operator bool() const { return get() != nullptr; }
261
262 // Returns false if the WeakPtr is confirmed to be invalid. This call is safe
263 // to make from any thread, e.g. to optimize away unnecessary work, but
264 // operator bool() must always be called, on the correct sequence, before
265 // actually using the pointer.
266 //
267 // Warning: as with any object, this call is only thread-safe if the WeakPtr
268 // instance isn't being re-assigned or reset() racily with this call.
MaybeValid()269 bool MaybeValid() const { return ref_.MaybeValid(); }
270
271 // Returns whether the object |this| points to has been invalidated. This can
272 // be used to distinguish a WeakPtr to a destroyed object from one that has
273 // been explicitly set to null.
WasInvalidated()274 bool WasInvalidated() const { return ptr_ && !ref_.IsValid(); }
275
276 private:
277 friend class internal::SupportsWeakPtrBase;
278 template <typename U> friend class WeakPtr;
279 friend class SupportsWeakPtr<T>;
280 friend class WeakPtrFactory<T>;
281
WeakPtr(const internal::WeakReference & ref,T * ptr)282 WeakPtr(const internal::WeakReference& ref, T* ptr)
283 : WeakPtrBase(ref, reinterpret_cast<uintptr_t>(ptr)) {}
284 };
285
286 // Allow callers to compare WeakPtrs against nullptr to test validity.
287 template <class T>
288 bool operator!=(const WeakPtr<T>& weak_ptr, std::nullptr_t) {
289 return !(weak_ptr == nullptr);
290 }
291 template <class T>
292 bool operator!=(std::nullptr_t, const WeakPtr<T>& weak_ptr) {
293 return weak_ptr != nullptr;
294 }
295 template <class T>
296 bool operator==(const WeakPtr<T>& weak_ptr, std::nullptr_t) {
297 return weak_ptr.get() == nullptr;
298 }
299 template <class T>
300 bool operator==(std::nullptr_t, const WeakPtr<T>& weak_ptr) {
301 return weak_ptr == nullptr;
302 }
303
304 namespace internal {
305 class BASE_EXPORT WeakPtrFactoryBase {
306 protected:
307 WeakPtrFactoryBase(uintptr_t ptr);
308 ~WeakPtrFactoryBase();
309 internal::WeakReferenceOwner weak_reference_owner_;
310 uintptr_t ptr_;
311 };
312 } // namespace internal
313
314 // A class may be composed of a WeakPtrFactory and thereby
315 // control how it exposes weak pointers to itself. This is helpful if you only
316 // need weak pointers within the implementation of a class. This class is also
317 // useful when working with primitive types. For example, you could have a
318 // WeakPtrFactory<bool> that is used to pass around a weak reference to a bool.
319 template <class T>
320 class WeakPtrFactory : public internal::WeakPtrFactoryBase {
321 public:
WeakPtrFactory(T * ptr)322 explicit WeakPtrFactory(T* ptr)
323 : WeakPtrFactoryBase(reinterpret_cast<uintptr_t>(ptr)) {}
324
325 ~WeakPtrFactory() = default;
326
GetWeakPtr()327 WeakPtr<T> GetWeakPtr() const {
328 return WeakPtr<T>(weak_reference_owner_.GetRef(),
329 reinterpret_cast<T*>(ptr_));
330 }
331
332 // Call this method to invalidate all existing weak pointers.
InvalidateWeakPtrs()333 void InvalidateWeakPtrs() {
334 DCHECK(ptr_);
335 weak_reference_owner_.Invalidate();
336 }
337
338 // Call this method to determine if any weak pointers exist.
HasWeakPtrs()339 bool HasWeakPtrs() const {
340 DCHECK(ptr_);
341 return weak_reference_owner_.HasRefs();
342 }
343
344 private:
345 DISALLOW_IMPLICIT_CONSTRUCTORS(WeakPtrFactory);
346 };
347
348 // A class may extend from SupportsWeakPtr to let others take weak pointers to
349 // it. This avoids the class itself implementing boilerplate to dispense weak
350 // pointers. However, since SupportsWeakPtr's destructor won't invalidate
351 // weak pointers to the class until after the derived class' members have been
352 // destroyed, its use can lead to subtle use-after-destroy issues.
353 template <class T>
354 class SupportsWeakPtr : public internal::SupportsWeakPtrBase {
355 public:
356 SupportsWeakPtr() = default;
357
AsWeakPtr()358 WeakPtr<T> AsWeakPtr() {
359 return WeakPtr<T>(weak_reference_owner_.GetRef(), static_cast<T*>(this));
360 }
361
362 protected:
363 ~SupportsWeakPtr() = default;
364
365 private:
366 internal::WeakReferenceOwner weak_reference_owner_;
367 DISALLOW_COPY_AND_ASSIGN(SupportsWeakPtr);
368 };
369
370 // Helper function that uses type deduction to safely return a WeakPtr<Derived>
371 // when Derived doesn't directly extend SupportsWeakPtr<Derived>, instead it
372 // extends a Base that extends SupportsWeakPtr<Base>.
373 //
374 // EXAMPLE:
375 // class Base : public base::SupportsWeakPtr<Producer> {};
376 // class Derived : public Base {};
377 //
378 // Derived derived;
379 // base::WeakPtr<Derived> ptr = base::AsWeakPtr(&derived);
380 //
381 // Note that the following doesn't work (invalid type conversion) since
382 // Derived::AsWeakPtr() is WeakPtr<Base> SupportsWeakPtr<Base>::AsWeakPtr(),
383 // and there's no way to safely cast WeakPtr<Base> to WeakPtr<Derived> at
384 // the caller.
385 //
386 // base::WeakPtr<Derived> ptr = derived.AsWeakPtr(); // Fails.
387
388 template <typename Derived>
AsWeakPtr(Derived * t)389 WeakPtr<Derived> AsWeakPtr(Derived* t) {
390 return internal::SupportsWeakPtrBase::StaticAsWeakPtr<Derived>(t);
391 }
392
393 } // namespace base
394
395 #endif // BASE_MEMORY_WEAK_PTR_H_
396