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