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 NULL) 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 "base/base_export.h" 74#include "base/logging.h" 75#include "base/macros.h" 76#include "base/memory/ref_counted.h" 77#include "base/sequence_checker.h" 78#include "base/template_util.h" 79 80namespace base { 81 82template <typename T> class SupportsWeakPtr; 83template <typename T> class WeakPtr; 84 85namespace internal { 86// These classes are part of the WeakPtr implementation. 87// DO NOT USE THESE CLASSES DIRECTLY YOURSELF. 88 89class BASE_EXPORT WeakReference { 90 public: 91 // Although Flag is bound to a specific SequencedTaskRunner, it may be 92 // deleted from another via base::WeakPtr::~WeakPtr(). 93 class BASE_EXPORT Flag : public RefCountedThreadSafe<Flag> { 94 public: 95 Flag(); 96 97 void Invalidate(); 98 bool IsValid() const; 99 100 private: 101 friend class base::RefCountedThreadSafe<Flag>; 102 103 ~Flag(); 104 105 SequenceChecker sequence_checker_; 106 bool is_valid_; 107 }; 108 109 WeakReference(); 110 explicit WeakReference(const Flag* flag); 111 ~WeakReference(); 112 113 bool is_valid() const; 114 115 private: 116 scoped_refptr<const Flag> flag_; 117}; 118 119class BASE_EXPORT WeakReferenceOwner { 120 public: 121 WeakReferenceOwner(); 122 ~WeakReferenceOwner(); 123 124 WeakReference GetRef() const; 125 126 bool HasRefs() const { 127 return flag_.get() && !flag_->HasOneRef(); 128 } 129 130 void Invalidate(); 131 132 private: 133 mutable scoped_refptr<WeakReference::Flag> flag_; 134}; 135 136// This class simplifies the implementation of WeakPtr's type conversion 137// constructor by avoiding the need for a public accessor for ref_. A 138// WeakPtr<T> cannot access the private members of WeakPtr<U>, so this 139// base class gives us a way to access ref_ in a protected fashion. 140class BASE_EXPORT WeakPtrBase { 141 public: 142 WeakPtrBase(); 143 ~WeakPtrBase(); 144 145 protected: 146 explicit WeakPtrBase(const WeakReference& ref); 147 148 WeakReference ref_; 149}; 150 151// This class provides a common implementation of common functions that would 152// otherwise get instantiated separately for each distinct instantiation of 153// SupportsWeakPtr<>. 154class SupportsWeakPtrBase { 155 public: 156 // A safe static downcast of a WeakPtr<Base> to WeakPtr<Derived>. This 157 // conversion will only compile if there is exists a Base which inherits 158 // from SupportsWeakPtr<Base>. See base::AsWeakPtr() below for a helper 159 // function that makes calling this easier. 160 template<typename Derived> 161 static WeakPtr<Derived> StaticAsWeakPtr(Derived* t) { 162 typedef 163 is_convertible<Derived, internal::SupportsWeakPtrBase&> convertible; 164 static_assert(convertible::value, 165 "AsWeakPtr argument must inherit from SupportsWeakPtr"); 166 return AsWeakPtrImpl<Derived>(t, *t); 167 } 168 169 private: 170 // This template function uses type inference to find a Base of Derived 171 // which is an instance of SupportsWeakPtr<Base>. We can then safely 172 // static_cast the Base* to a Derived*. 173 template <typename Derived, typename Base> 174 static WeakPtr<Derived> AsWeakPtrImpl( 175 Derived* t, const SupportsWeakPtr<Base>&) { 176 WeakPtr<Base> ptr = t->Base::AsWeakPtr(); 177 return WeakPtr<Derived>(ptr.ref_, static_cast<Derived*>(ptr.ptr_)); 178 } 179}; 180 181} // namespace internal 182 183template <typename T> class WeakPtrFactory; 184 185// The WeakPtr class holds a weak reference to |T*|. 186// 187// This class is designed to be used like a normal pointer. You should always 188// null-test an object of this class before using it or invoking a method that 189// may result in the underlying object being destroyed. 190// 191// EXAMPLE: 192// 193// class Foo { ... }; 194// WeakPtr<Foo> foo; 195// if (foo) 196// foo->method(); 197// 198template <typename T> 199class WeakPtr : public internal::WeakPtrBase { 200 public: 201 WeakPtr() : ptr_(NULL) { 202 } 203 204 // Allow conversion from U to T provided U "is a" T. Note that this 205 // is separate from the (implicit) copy constructor. 206 template <typename U> 207 WeakPtr(const WeakPtr<U>& other) : WeakPtrBase(other), ptr_(other.ptr_) { 208 } 209 210 T* get() const { return ref_.is_valid() ? ptr_ : NULL; } 211 212 T& operator*() const { 213 DCHECK(get() != NULL); 214 return *get(); 215 } 216 T* operator->() const { 217 DCHECK(get() != NULL); 218 return get(); 219 } 220 221 // Allow WeakPtr<element_type> to be used in boolean expressions, but not 222 // implicitly convertible to a real bool (which is dangerous). 223 // 224 // Note that this trick is only safe when the == and != operators 225 // are declared explicitly, as otherwise "weak_ptr1 == weak_ptr2" 226 // will compile but do the wrong thing (i.e., convert to Testable 227 // and then do the comparison). 228 private: 229 typedef T* WeakPtr::*Testable; 230 231 public: 232 operator Testable() const { return get() ? &WeakPtr::ptr_ : NULL; } 233 234 void reset() { 235 ref_ = internal::WeakReference(); 236 ptr_ = NULL; 237 } 238 239 private: 240 // Explicitly declare comparison operators as required by the bool 241 // trick, but keep them private. 242 template <class U> bool operator==(WeakPtr<U> const&) const; 243 template <class U> bool operator!=(WeakPtr<U> const&) const; 244 245 friend class internal::SupportsWeakPtrBase; 246 template <typename U> friend class WeakPtr; 247 friend class SupportsWeakPtr<T>; 248 friend class WeakPtrFactory<T>; 249 250 WeakPtr(const internal::WeakReference& ref, T* ptr) 251 : WeakPtrBase(ref), 252 ptr_(ptr) { 253 } 254 255 // This pointer is only valid when ref_.is_valid() is true. Otherwise, its 256 // value is undefined (as opposed to NULL). 257 T* ptr_; 258}; 259 260// A class may be composed of a WeakPtrFactory and thereby 261// control how it exposes weak pointers to itself. This is helpful if you only 262// need weak pointers within the implementation of a class. This class is also 263// useful when working with primitive types. For example, you could have a 264// WeakPtrFactory<bool> that is used to pass around a weak reference to a bool. 265template <class T> 266class WeakPtrFactory { 267 public: 268 explicit WeakPtrFactory(T* ptr) : ptr_(ptr) { 269 } 270 271 ~WeakPtrFactory() { 272 ptr_ = NULL; 273 } 274 275 WeakPtr<T> GetWeakPtr() { 276 DCHECK(ptr_); 277 return WeakPtr<T>(weak_reference_owner_.GetRef(), ptr_); 278 } 279 280 // Call this method to invalidate all existing weak pointers. 281 void InvalidateWeakPtrs() { 282 DCHECK(ptr_); 283 weak_reference_owner_.Invalidate(); 284 } 285 286 // Call this method to determine if any weak pointers exist. 287 bool HasWeakPtrs() const { 288 DCHECK(ptr_); 289 return weak_reference_owner_.HasRefs(); 290 } 291 292 private: 293 internal::WeakReferenceOwner weak_reference_owner_; 294 T* ptr_; 295 DISALLOW_IMPLICIT_CONSTRUCTORS(WeakPtrFactory); 296}; 297 298// A class may extend from SupportsWeakPtr to let others take weak pointers to 299// it. This avoids the class itself implementing boilerplate to dispense weak 300// pointers. However, since SupportsWeakPtr's destructor won't invalidate 301// weak pointers to the class until after the derived class' members have been 302// destroyed, its use can lead to subtle use-after-destroy issues. 303template <class T> 304class SupportsWeakPtr : public internal::SupportsWeakPtrBase { 305 public: 306 SupportsWeakPtr() {} 307 308 WeakPtr<T> AsWeakPtr() { 309 return WeakPtr<T>(weak_reference_owner_.GetRef(), static_cast<T*>(this)); 310 } 311 312 protected: 313 ~SupportsWeakPtr() {} 314 315 private: 316 internal::WeakReferenceOwner weak_reference_owner_; 317 DISALLOW_COPY_AND_ASSIGN(SupportsWeakPtr); 318}; 319 320// Helper function that uses type deduction to safely return a WeakPtr<Derived> 321// when Derived doesn't directly extend SupportsWeakPtr<Derived>, instead it 322// extends a Base that extends SupportsWeakPtr<Base>. 323// 324// EXAMPLE: 325// class Base : public base::SupportsWeakPtr<Producer> {}; 326// class Derived : public Base {}; 327// 328// Derived derived; 329// base::WeakPtr<Derived> ptr = base::AsWeakPtr(&derived); 330// 331// Note that the following doesn't work (invalid type conversion) since 332// Derived::AsWeakPtr() is WeakPtr<Base> SupportsWeakPtr<Base>::AsWeakPtr(), 333// and there's no way to safely cast WeakPtr<Base> to WeakPtr<Derived> at 334// the caller. 335// 336// base::WeakPtr<Derived> ptr = derived.AsWeakPtr(); // Fails. 337 338template <typename Derived> 339WeakPtr<Derived> AsWeakPtr(Derived* t) { 340 return internal::SupportsWeakPtrBase::StaticAsWeakPtr<Derived>(t); 341} 342 343} // namespace base 344 345#endif // BASE_MEMORY_WEAK_PTR_H_ 346