weak_ptr.h revision 90dce4d38c5ff5333bea97d859d4e484e27edf0c
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/template_util.h" 71#include "base/threading/thread_checker.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 // Remove this when crbug.com/234964 is addressed. 94 void DetachFromThreadHack() { thread_checker_.DetachFromThread(); } 95 96 private: 97 friend class base::RefCountedThreadSafe<Flag>; 98 99 ~Flag(); 100 101 ThreadChecker thread_checker_; 102 bool is_valid_; 103 }; 104 105 WeakReference(); 106 explicit WeakReference(const Flag* flag); 107 ~WeakReference(); 108 109 bool is_valid() const; 110 111 private: 112 scoped_refptr<const Flag> flag_; 113}; 114 115class BASE_EXPORT WeakReferenceOwner { 116 public: 117 WeakReferenceOwner(); 118 ~WeakReferenceOwner(); 119 120 WeakReference GetRef() const; 121 122 bool HasRefs() const { 123 return flag_.get() && !flag_->HasOneRef(); 124 } 125 126 void Invalidate(); 127 128 // Remove this when crbug.com/234964 is addressed. 129 void DetachFromThreadHack() { 130 if (flag_) flag_->DetachFromThreadHack(); 131 } 132 133 private: 134 mutable scoped_refptr<WeakReference::Flag> flag_; 135}; 136 137// This class simplifies the implementation of WeakPtr's type conversion 138// constructor by avoiding the need for a public accessor for ref_. A 139// WeakPtr<T> cannot access the private members of WeakPtr<U>, so this 140// base class gives us a way to access ref_ in a protected fashion. 141class BASE_EXPORT WeakPtrBase { 142 public: 143 WeakPtrBase(); 144 ~WeakPtrBase(); 145 146 protected: 147 explicit WeakPtrBase(const WeakReference& ref); 148 149 WeakReference ref_; 150}; 151 152// This class provides a common implementation of common functions that would 153// otherwise get instantiated separately for each distinct instantiation of 154// SupportsWeakPtr<>. 155class SupportsWeakPtrBase { 156 public: 157 // A safe static downcast of a WeakPtr<Base> to WeakPtr<Derived>. This 158 // conversion will only compile if there is exists a Base which inherits 159 // from SupportsWeakPtr<Base>. See base::AsWeakPtr() below for a helper 160 // function that makes calling this easier. 161 template<typename Derived> 162 static WeakPtr<Derived> StaticAsWeakPtr(Derived* t) { 163 typedef 164 is_convertible<Derived, internal::SupportsWeakPtrBase&> convertible; 165 COMPILE_ASSERT(convertible::value, 166 AsWeakPtr_argument_inherits_from_SupportsWeakPtr); 167 return AsWeakPtrImpl<Derived>(t, *t); 168 } 169 170 private: 171 // This template function uses type inference to find a Base of Derived 172 // which is an instance of SupportsWeakPtr<Base>. We can then safely 173 // static_cast the Base* to a Derived*. 174 template <typename Derived, typename Base> 175 static WeakPtr<Derived> AsWeakPtrImpl( 176 Derived* t, const SupportsWeakPtr<Base>&) { 177 WeakPtr<Base> ptr = t->Base::AsWeakPtr(); 178 return WeakPtr<Derived>(ptr.ref_, static_cast<Derived*>(ptr.ptr_)); 179 } 180}; 181 182} // namespace internal 183 184template <typename T> class WeakPtrFactory; 185 186// The WeakPtr class holds a weak reference to |T*|. 187// 188// This class is designed to be used like a normal pointer. You should always 189// null-test an object of this class before using it or invoking a method that 190// may result in the underlying object being destroyed. 191// 192// EXAMPLE: 193// 194// class Foo { ... }; 195// WeakPtr<Foo> foo; 196// if (foo) 197// foo->method(); 198// 199template <typename T> 200class WeakPtr : public internal::WeakPtrBase { 201 public: 202 WeakPtr() : ptr_(NULL) { 203 } 204 205 // Allow conversion from U to T provided U "is a" T. Note that this 206 // is separate from the (implicit) copy constructor. 207 template <typename U> 208 WeakPtr(const WeakPtr<U>& other) : WeakPtrBase(other), ptr_(other.ptr_) { 209 } 210 211 T* get() const { return ref_.is_valid() ? ptr_ : NULL; } 212 operator T*() const { return get(); } 213 214 T& operator*() const { 215 DCHECK(get() != NULL); 216 return *get(); 217 } 218 T* operator->() const { 219 DCHECK(get() != NULL); 220 return get(); 221 } 222 223 void reset() { 224 ref_ = internal::WeakReference(); 225 ptr_ = NULL; 226 } 227 228 private: 229 friend class internal::SupportsWeakPtrBase; 230 template <typename U> friend class WeakPtr; 231 friend class SupportsWeakPtr<T>; 232 friend class WeakPtrFactory<T>; 233 234 WeakPtr(const internal::WeakReference& ref, T* ptr) 235 : WeakPtrBase(ref), 236 ptr_(ptr) { 237 } 238 239 // This pointer is only valid when ref_.is_valid() is true. Otherwise, its 240 // value is undefined (as opposed to NULL). 241 T* ptr_; 242}; 243 244// A class may be composed of a WeakPtrFactory and thereby 245// control how it exposes weak pointers to itself. This is helpful if you only 246// need weak pointers within the implementation of a class. This class is also 247// useful when working with primitive types. For example, you could have a 248// WeakPtrFactory<bool> that is used to pass around a weak reference to a bool. 249template <class T> 250class WeakPtrFactory { 251 public: 252 explicit WeakPtrFactory(T* ptr) : ptr_(ptr) { 253 } 254 255 ~WeakPtrFactory() { 256 ptr_ = NULL; 257 } 258 259 WeakPtr<T> GetWeakPtr() { 260 DCHECK(ptr_); 261 return WeakPtr<T>(weak_reference_owner_.GetRef(), ptr_); 262 } 263 264 // Call this method to invalidate all existing weak pointers. 265 void InvalidateWeakPtrs() { 266 DCHECK(ptr_); 267 weak_reference_owner_.Invalidate(); 268 } 269 270 // Call this method to determine if any weak pointers exist. 271 bool HasWeakPtrs() const { 272 DCHECK(ptr_); 273 return weak_reference_owner_.HasRefs(); 274 } 275 276 private: 277 internal::WeakReferenceOwner weak_reference_owner_; 278 T* ptr_; 279 DISALLOW_IMPLICIT_CONSTRUCTORS(WeakPtrFactory); 280}; 281 282// A class may extend from SupportsWeakPtr to let others take weak pointers to 283// it. This avoids the class itself implementing boilerplate to dispense weak 284// pointers. However, since SupportsWeakPtr's destructor won't invalidate 285// weak pointers to the class until after the derived class' members have been 286// destroyed, its use can lead to subtle use-after-destroy issues. 287template <class T> 288class SupportsWeakPtr : public internal::SupportsWeakPtrBase { 289 public: 290 SupportsWeakPtr() {} 291 292 WeakPtr<T> AsWeakPtr() { 293 return WeakPtr<T>(weak_reference_owner_.GetRef(), static_cast<T*>(this)); 294 } 295 296 // Removes the binding, if any, from this object to a particular thread. 297 // This is used in WebGraphicsContext3DInProcessCommandBufferImpl to work- 298 // around access to cmmand buffer objects by more than one thread. 299 // Remove this when crbug.com/234964 is addressed. 300 void DetachFromThreadHack() { 301 weak_reference_owner_.DetachFromThreadHack(); 302 } 303 304 protected: 305 ~SupportsWeakPtr() {} 306 307 private: 308 internal::WeakReferenceOwner weak_reference_owner_; 309 DISALLOW_COPY_AND_ASSIGN(SupportsWeakPtr); 310}; 311 312// Helper function that uses type deduction to safely return a WeakPtr<Derived> 313// when Derived doesn't directly extend SupportsWeakPtr<Derived>, instead it 314// extends a Base that extends SupportsWeakPtr<Base>. 315// 316// EXAMPLE: 317// class Base : public base::SupportsWeakPtr<Producer> {}; 318// class Derived : public Base {}; 319// 320// Derived derived; 321// base::WeakPtr<Derived> ptr = base::AsWeakPtr(&derived); 322// 323// Note that the following doesn't work (invalid type conversion) since 324// Derived::AsWeakPtr() is WeakPtr<Base> SupportsWeakPtr<Base>::AsWeakPtr(), 325// and there's no way to safely cast WeakPtr<Base> to WeakPtr<Derived> at 326// the caller. 327// 328// base::WeakPtr<Derived> ptr = derived.AsWeakPtr(); // Fails. 329 330template <typename Derived> 331WeakPtr<Derived> AsWeakPtr(Derived* t) { 332 return internal::SupportsWeakPtrBase::StaticAsWeakPtr<Derived>(t); 333} 334 335} // namespace base 336 337#endif // BASE_MEMORY_WEAK_PTR_H_ 338