weak_ptr.h revision a36e5920737c6adbddd3e43b760e5de8431db6e0
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// 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_; 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 threads, but must always be 51// dereferenced and invalidated on the same thread otherwise checking the 52// 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// thread, and cannot be dereferenced or invalidated on any other thread. Bound 57// WeakPtrs can still be handed off to other threads, e.g. to use to post tasks 58// back to object on the bound thread. 59// 60// Invalidating the factory's WeakPtrs un-binds it from the thread, allowing it 61// to be passed for a different thread to use or delete it. 62 63#ifndef BASE_MEMORY_WEAK_PTR_H_ 64#define BASE_MEMORY_WEAK_PTR_H_ 65 66#include "base/basictypes.h" 67#include "base/base_export.h" 68#include "base/logging.h" 69#include "base/memory/ref_counted.h" 70#include "base/sequence_checker.h" 71#include "base/template_util.h" 72 73namespace base { 74 75template <typename T> class SupportsWeakPtr; 76template <typename T> class WeakPtr; 77 78namespace internal { 79// These classes are part of the WeakPtr implementation. 80// DO NOT USE THESE CLASSES DIRECTLY YOURSELF. 81 82class BASE_EXPORT WeakReference { 83 public: 84 // Although Flag is bound to a specific thread, it may be deleted from another 85 // via base::WeakPtr::~WeakPtr(). 86 class Flag : public RefCountedThreadSafe<Flag> { 87 public: 88 Flag(); 89 90 void Invalidate(); 91 bool IsValid() const; 92 93 private: 94 friend class base::RefCountedThreadSafe<Flag>; 95 96 ~Flag(); 97 98 SequenceChecker sequence_checker_; 99 bool is_valid_; 100 }; 101 102 WeakReference(); 103 explicit WeakReference(const Flag* flag); 104 ~WeakReference(); 105 106 bool is_valid() const; 107 108 private: 109 scoped_refptr<const Flag> flag_; 110}; 111 112class BASE_EXPORT WeakReferenceOwner { 113 public: 114 WeakReferenceOwner(); 115 ~WeakReferenceOwner(); 116 117 WeakReference GetRef() const; 118 119 bool HasRefs() const { 120 return flag_.get() && !flag_->HasOneRef(); 121 } 122 123 void Invalidate(); 124 125 private: 126 mutable scoped_refptr<WeakReference::Flag> flag_; 127}; 128 129// This class simplifies the implementation of WeakPtr's type conversion 130// constructor by avoiding the need for a public accessor for ref_. A 131// WeakPtr<T> cannot access the private members of WeakPtr<U>, so this 132// base class gives us a way to access ref_ in a protected fashion. 133class BASE_EXPORT WeakPtrBase { 134 public: 135 WeakPtrBase(); 136 ~WeakPtrBase(); 137 138 protected: 139 explicit WeakPtrBase(const WeakReference& ref); 140 141 WeakReference ref_; 142}; 143 144// This class provides a common implementation of common functions that would 145// otherwise get instantiated separately for each distinct instantiation of 146// SupportsWeakPtr<>. 147class SupportsWeakPtrBase { 148 public: 149 // A safe static downcast of a WeakPtr<Base> to WeakPtr<Derived>. This 150 // conversion will only compile if there is exists a Base which inherits 151 // from SupportsWeakPtr<Base>. See base::AsWeakPtr() below for a helper 152 // function that makes calling this easier. 153 template<typename Derived> 154 static WeakPtr<Derived> StaticAsWeakPtr(Derived* t) { 155 typedef 156 is_convertible<Derived, internal::SupportsWeakPtrBase&> convertible; 157 COMPILE_ASSERT(convertible::value, 158 AsWeakPtr_argument_inherits_from_SupportsWeakPtr); 159 return AsWeakPtrImpl<Derived>(t, *t); 160 } 161 162 private: 163 // This template function uses type inference to find a Base of Derived 164 // which is an instance of SupportsWeakPtr<Base>. We can then safely 165 // static_cast the Base* to a Derived*. 166 template <typename Derived, typename Base> 167 static WeakPtr<Derived> AsWeakPtrImpl( 168 Derived* t, const SupportsWeakPtr<Base>&) { 169 WeakPtr<Base> ptr = t->Base::AsWeakPtr(); 170 return WeakPtr<Derived>(ptr.ref_, static_cast<Derived*>(ptr.ptr_)); 171 } 172}; 173 174} // namespace internal 175 176template <typename T> class WeakPtrFactory; 177 178// The WeakPtr class holds a weak reference to |T*|. 179// 180// This class is designed to be used like a normal pointer. You should always 181// null-test an object of this class before using it or invoking a method that 182// may result in the underlying object being destroyed. 183// 184// EXAMPLE: 185// 186// class Foo { ... }; 187// WeakPtr<Foo> foo; 188// if (foo) 189// foo->method(); 190// 191template <typename T> 192class WeakPtr : public internal::WeakPtrBase { 193 public: 194 WeakPtr() : ptr_(NULL) { 195 } 196 197 // Allow conversion from U to T provided U "is a" T. Note that this 198 // is separate from the (implicit) copy constructor. 199 template <typename U> 200 WeakPtr(const WeakPtr<U>& other) : WeakPtrBase(other), ptr_(other.ptr_) { 201 } 202 203 T* get() const { return ref_.is_valid() ? ptr_ : NULL; } 204 205 T& operator*() const { 206 DCHECK(get() != NULL); 207 return *get(); 208 } 209 T* operator->() const { 210 DCHECK(get() != NULL); 211 return get(); 212 } 213 214 // Allow WeakPtr<element_type> to be used in boolean expressions, but not 215 // implicitly convertible to a real bool (which is dangerous). 216 // 217 // Note that this trick is only safe when the == and != operators 218 // are declared explicitly, as otherwise "weak_ptr1 == weak_ptr2" 219 // will compile but do the wrong thing (i.e., convert to Testable 220 // and then do the comparison). 221 private: 222 typedef T* WeakPtr::*Testable; 223 224 public: 225 operator Testable() const { return get() ? &WeakPtr::ptr_ : NULL; } 226 227 void reset() { 228 ref_ = internal::WeakReference(); 229 ptr_ = NULL; 230 } 231 232 private: 233 // Explicitly declare comparison operators as required by the bool 234 // trick, but keep them private. 235 template <class U> bool operator==(WeakPtr<U> const&) const; 236 template <class U> bool operator!=(WeakPtr<U> const&) const; 237 238 friend class internal::SupportsWeakPtrBase; 239 template <typename U> friend class WeakPtr; 240 friend class SupportsWeakPtr<T>; 241 friend class WeakPtrFactory<T>; 242 243 WeakPtr(const internal::WeakReference& ref, T* ptr) 244 : WeakPtrBase(ref), 245 ptr_(ptr) { 246 } 247 248 // This pointer is only valid when ref_.is_valid() is true. Otherwise, its 249 // value is undefined (as opposed to NULL). 250 T* ptr_; 251}; 252 253// A class may be composed of a WeakPtrFactory and thereby 254// control how it exposes weak pointers to itself. This is helpful if you only 255// need weak pointers within the implementation of a class. This class is also 256// useful when working with primitive types. For example, you could have a 257// WeakPtrFactory<bool> that is used to pass around a weak reference to a bool. 258template <class T> 259class WeakPtrFactory { 260 public: 261 explicit WeakPtrFactory(T* ptr) : ptr_(ptr) { 262 } 263 264 ~WeakPtrFactory() { 265 ptr_ = NULL; 266 } 267 268 WeakPtr<T> GetWeakPtr() { 269 DCHECK(ptr_); 270 return WeakPtr<T>(weak_reference_owner_.GetRef(), ptr_); 271 } 272 273 // Call this method to invalidate all existing weak pointers. 274 void InvalidateWeakPtrs() { 275 DCHECK(ptr_); 276 weak_reference_owner_.Invalidate(); 277 } 278 279 // Call this method to determine if any weak pointers exist. 280 bool HasWeakPtrs() const { 281 DCHECK(ptr_); 282 return weak_reference_owner_.HasRefs(); 283 } 284 285 private: 286 internal::WeakReferenceOwner weak_reference_owner_; 287 T* ptr_; 288 DISALLOW_IMPLICIT_CONSTRUCTORS(WeakPtrFactory); 289}; 290 291// A class may extend from SupportsWeakPtr to let others take weak pointers to 292// it. This avoids the class itself implementing boilerplate to dispense weak 293// pointers. However, since SupportsWeakPtr's destructor won't invalidate 294// weak pointers to the class until after the derived class' members have been 295// destroyed, its use can lead to subtle use-after-destroy issues. 296template <class T> 297class SupportsWeakPtr : public internal::SupportsWeakPtrBase { 298 public: 299 SupportsWeakPtr() {} 300 301 WeakPtr<T> AsWeakPtr() { 302 return WeakPtr<T>(weak_reference_owner_.GetRef(), static_cast<T*>(this)); 303 } 304 305 protected: 306 ~SupportsWeakPtr() {} 307 308 private: 309 internal::WeakReferenceOwner weak_reference_owner_; 310 DISALLOW_COPY_AND_ASSIGN(SupportsWeakPtr); 311}; 312 313// Helper function that uses type deduction to safely return a WeakPtr<Derived> 314// when Derived doesn't directly extend SupportsWeakPtr<Derived>, instead it 315// extends a Base that extends SupportsWeakPtr<Base>. 316// 317// EXAMPLE: 318// class Base : public base::SupportsWeakPtr<Producer> {}; 319// class Derived : public Base {}; 320// 321// Derived derived; 322// base::WeakPtr<Derived> ptr = base::AsWeakPtr(&derived); 323// 324// Note that the following doesn't work (invalid type conversion) since 325// Derived::AsWeakPtr() is WeakPtr<Base> SupportsWeakPtr<Base>::AsWeakPtr(), 326// and there's no way to safely cast WeakPtr<Base> to WeakPtr<Derived> at 327// the caller. 328// 329// base::WeakPtr<Derived> ptr = derived.AsWeakPtr(); // Fails. 330 331template <typename Derived> 332WeakPtr<Derived> AsWeakPtr(Derived* t) { 333 return internal::SupportsWeakPtrBase::StaticAsWeakPtr<Derived>(t); 334} 335 336} // namespace base 337 338#endif // BASE_MEMORY_WEAK_PTR_H_ 339