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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