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.
5 // Weak pointers are pointers to an object that do not affect its lifetime,
6 // and which may be invalidated (i.e. reset to NULL) by the object, or its
7 // owner, at any time, most commonly when the object is about to be deleted.
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.
19 // void SpawnWorker() { Worker::StartNew(weak_factory_.GetWeakPtr()); }
20 // void WorkComplete(const Result& result) { ... }
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_;
30 // static void StartNew(const WeakPtr<Controller>& controller) {
31 // Worker* worker = new Worker(controller);
32 // // Kick off asynchronous processing...
35 // Worker(const WeakPtr<Controller>& controller)
36 // : controller_(controller) {}
37 // void DidCompleteAsynchronousProcessing(const Result& result) {
39 // controller_->WorkComplete(result);
41 // WeakPtr<Controller> controller_;
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.
48 // ------------------------- IMPORTANT: Thread-safety -------------------------
50 // Weak pointers may be passed safely between threads, but must always be
51 // dereferenced and invalidated on the same SequencedTaskRunner otherwise
52 // checking the pointer would be racey.
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 // thread 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
61 // Invalidating the factory's WeakPtrs un-binds it from the sequence, allowing
62 // it to be passed for a different sequence to use or delete it.
64 #ifndef BASE_MEMORY_WEAK_PTR_H_
65 #define BASE_MEMORY_WEAK_PTR_H_
67 #include "base/basictypes.h"
68 #include "base/base_export.h"
69 #include "base/logging.h"
70 #include "base/memory/ref_counted.h"
71 #include "base/sequence_checker.h"
72 #include "base/template_util.h"
76 template <typename T> class SupportsWeakPtr;
77 template <typename T> class WeakPtr;
80 // These classes are part of the WeakPtr implementation.
81 // DO NOT USE THESE CLASSES DIRECTLY YOURSELF.
83 class BASE_EXPORT WeakReference {
85 // Although Flag is bound to a specific SequencedTaskRunner, it may be
86 // deleted from another via base::WeakPtr::~WeakPtr().
87 class BASE_EXPORT Flag : public RefCountedThreadSafe<Flag> {
95 friend class base::RefCountedThreadSafe<Flag>;
99 SequenceChecker sequence_checker_;
104 explicit WeakReference(const Flag* flag);
107 bool is_valid() const;
110 scoped_refptr<const Flag> flag_;
113 class BASE_EXPORT WeakReferenceOwner {
115 WeakReferenceOwner();
116 ~WeakReferenceOwner();
118 WeakReference GetRef() const;
120 bool HasRefs() const {
121 return flag_.get() && !flag_->HasOneRef();
127 mutable scoped_refptr<WeakReference::Flag> flag_;
130 // This class simplifies the implementation of WeakPtr's type conversion
131 // constructor by avoiding the need for a public accessor for ref_. A
132 // WeakPtr<T> cannot access the private members of WeakPtr<U>, so this
133 // base class gives us a way to access ref_ in a protected fashion.
134 class BASE_EXPORT WeakPtrBase {
140 explicit WeakPtrBase(const WeakReference& ref);
145 // This class provides a common implementation of common functions that would
146 // otherwise get instantiated separately for each distinct instantiation of
147 // SupportsWeakPtr<>.
148 class SupportsWeakPtrBase {
150 // A safe static downcast of a WeakPtr<Base> to WeakPtr<Derived>. This
151 // conversion will only compile if there is exists a Base which inherits
152 // from SupportsWeakPtr<Base>. See base::AsWeakPtr() below for a helper
153 // function that makes calling this easier.
154 template<typename Derived>
155 static WeakPtr<Derived> StaticAsWeakPtr(Derived* t) {
157 is_convertible<Derived, internal::SupportsWeakPtrBase&> convertible;
158 COMPILE_ASSERT(convertible::value,
159 AsWeakPtr_argument_inherits_from_SupportsWeakPtr);
160 return AsWeakPtrImpl<Derived>(t, *t);
164 // This template function uses type inference to find a Base of Derived
165 // which is an instance of SupportsWeakPtr<Base>. We can then safely
166 // static_cast the Base* to a Derived*.
167 template <typename Derived, typename Base>
168 static WeakPtr<Derived> AsWeakPtrImpl(
169 Derived* t, const SupportsWeakPtr<Base>&) {
170 WeakPtr<Base> ptr = t->Base::AsWeakPtr();
171 return WeakPtr<Derived>(ptr.ref_, static_cast<Derived*>(ptr.ptr_));
175 } // namespace internal
177 template <typename T> class WeakPtrFactory;
179 // The WeakPtr class holds a weak reference to |T*|.
181 // This class is designed to be used like a normal pointer. You should always
182 // null-test an object of this class before using it or invoking a method that
183 // may result in the underlying object being destroyed.
187 // class Foo { ... };
192 template <typename T>
193 class WeakPtr : public internal::WeakPtrBase {
195 WeakPtr() : ptr_(NULL) {
198 // Allow conversion from U to T provided U "is a" T. Note that this
199 // is separate from the (implicit) copy constructor.
200 template <typename U>
201 WeakPtr(const WeakPtr<U>& other) : WeakPtrBase(other), ptr_(other.ptr_) {
204 T* get() const { return ref_.is_valid() ? ptr_ : NULL; }
206 T& operator*() const {
207 DCHECK(get() != NULL);
210 T* operator->() const {
211 DCHECK(get() != NULL);
215 // Allow WeakPtr<element_type> to be used in boolean expressions, but not
216 // implicitly convertible to a real bool (which is dangerous).
218 // Note that this trick is only safe when the == and != operators
219 // are declared explicitly, as otherwise "weak_ptr1 == weak_ptr2"
220 // will compile but do the wrong thing (i.e., convert to Testable
221 // and then do the comparison).
223 typedef T* WeakPtr::*Testable;
226 operator Testable() const { return get() ? &WeakPtr::ptr_ : NULL; }
229 ref_ = internal::WeakReference();
234 // Explicitly declare comparison operators as required by the bool
235 // trick, but keep them private.
236 template <class U> bool operator==(WeakPtr<U> const&) const;
237 template <class U> bool operator!=(WeakPtr<U> const&) const;
239 friend class internal::SupportsWeakPtrBase;
240 template <typename U> friend class WeakPtr;
241 friend class SupportsWeakPtr<T>;
242 friend class WeakPtrFactory<T>;
244 WeakPtr(const internal::WeakReference& ref, T* ptr)
249 // This pointer is only valid when ref_.is_valid() is true. Otherwise, its
250 // value is undefined (as opposed to NULL).
254 // A class may be composed of a WeakPtrFactory and thereby
255 // control how it exposes weak pointers to itself. This is helpful if you only
256 // need weak pointers within the implementation of a class. This class is also
257 // useful when working with primitive types. For example, you could have a
258 // WeakPtrFactory<bool> that is used to pass around a weak reference to a bool.
260 class WeakPtrFactory {
262 explicit WeakPtrFactory(T* ptr) : ptr_(ptr) {
269 WeakPtr<T> GetWeakPtr() {
271 return WeakPtr<T>(weak_reference_owner_.GetRef(), ptr_);
274 // Call this method to invalidate all existing weak pointers.
275 void InvalidateWeakPtrs() {
277 weak_reference_owner_.Invalidate();
280 // Call this method to determine if any weak pointers exist.
281 bool HasWeakPtrs() const {
283 return weak_reference_owner_.HasRefs();
287 internal::WeakReferenceOwner weak_reference_owner_;
289 DISALLOW_IMPLICIT_CONSTRUCTORS(WeakPtrFactory);
292 // A class may extend from SupportsWeakPtr to let others take weak pointers to
293 // it. This avoids the class itself implementing boilerplate to dispense weak
294 // pointers. However, since SupportsWeakPtr's destructor won't invalidate
295 // weak pointers to the class until after the derived class' members have been
296 // destroyed, its use can lead to subtle use-after-destroy issues.
298 class SupportsWeakPtr : public internal::SupportsWeakPtrBase {
302 WeakPtr<T> AsWeakPtr() {
303 return WeakPtr<T>(weak_reference_owner_.GetRef(), static_cast<T*>(this));
307 ~SupportsWeakPtr() {}
310 internal::WeakReferenceOwner weak_reference_owner_;
311 DISALLOW_COPY_AND_ASSIGN(SupportsWeakPtr);
314 // Helper function that uses type deduction to safely return a WeakPtr<Derived>
315 // when Derived doesn't directly extend SupportsWeakPtr<Derived>, instead it
316 // extends a Base that extends SupportsWeakPtr<Base>.
319 // class Base : public base::SupportsWeakPtr<Producer> {};
320 // class Derived : public Base {};
323 // base::WeakPtr<Derived> ptr = base::AsWeakPtr(&derived);
325 // Note that the following doesn't work (invalid type conversion) since
326 // Derived::AsWeakPtr() is WeakPtr<Base> SupportsWeakPtr<Base>::AsWeakPtr(),
327 // and there's no way to safely cast WeakPtr<Base> to WeakPtr<Derived> at
330 // base::WeakPtr<Derived> ptr = derived.AsWeakPtr(); // Fails.
332 template <typename Derived>
333 WeakPtr<Derived> AsWeakPtr(Derived* t) {
334 return internal::SupportsWeakPtrBase::StaticAsWeakPtr<Derived>(t);
339 #endif // BASE_MEMORY_WEAK_PTR_H_