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 nullptr) 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 <cstddef> 74#include <type_traits> 75 76#include "base/base_export.h" 77#include "base/logging.h" 78#include "base/macros.h" 79#include "base/memory/ref_counted.h" 80#include "base/sequence_checker.h" 81 82namespace base { 83 84template <typename T> class SupportsWeakPtr; 85template <typename T> class WeakPtr; 86 87namespace internal { 88// These classes are part of the WeakPtr implementation. 89// DO NOT USE THESE CLASSES DIRECTLY YOURSELF. 90 91class BASE_EXPORT WeakReference { 92 public: 93 // Although Flag is bound to a specific SequencedTaskRunner, it may be 94 // deleted from another via base::WeakPtr::~WeakPtr(). 95 class BASE_EXPORT Flag : public RefCountedThreadSafe<Flag> { 96 public: 97 Flag(); 98 99 void Invalidate(); 100 bool IsValid() const; 101 102 private: 103 friend class base::RefCountedThreadSafe<Flag>; 104 105 ~Flag(); 106 107 SequenceChecker sequence_checker_; 108 bool is_valid_; 109 }; 110 111 WeakReference(); 112 explicit WeakReference(const Flag* flag); 113 ~WeakReference(); 114 115 WeakReference(WeakReference&& other); 116 WeakReference(const WeakReference& other); 117 WeakReference& operator=(WeakReference&& other) = default; 118 WeakReference& operator=(const WeakReference& other) = default; 119 120 bool is_valid() const; 121 122 private: 123 scoped_refptr<const Flag> flag_; 124}; 125 126class BASE_EXPORT WeakReferenceOwner { 127 public: 128 WeakReferenceOwner(); 129 ~WeakReferenceOwner(); 130 131 WeakReference GetRef() const; 132 133 bool HasRefs() const { 134 return flag_.get() && !flag_->HasOneRef(); 135 } 136 137 void Invalidate(); 138 139 private: 140 mutable scoped_refptr<WeakReference::Flag> flag_; 141}; 142 143// This class simplifies the implementation of WeakPtr's type conversion 144// constructor by avoiding the need for a public accessor for ref_. A 145// WeakPtr<T> cannot access the private members of WeakPtr<U>, so this 146// base class gives us a way to access ref_ in a protected fashion. 147class BASE_EXPORT WeakPtrBase { 148 public: 149 WeakPtrBase(); 150 ~WeakPtrBase(); 151 152 WeakPtrBase(const WeakPtrBase& other) = default; 153 WeakPtrBase(WeakPtrBase&& other) = default; 154 WeakPtrBase& operator=(const WeakPtrBase& other) = default; 155 WeakPtrBase& operator=(WeakPtrBase&& other) = default; 156 157 protected: 158 explicit WeakPtrBase(const WeakReference& ref); 159 160 WeakReference ref_; 161}; 162 163// This class provides a common implementation of common functions that would 164// otherwise get instantiated separately for each distinct instantiation of 165// SupportsWeakPtr<>. 166class SupportsWeakPtrBase { 167 public: 168 // A safe static downcast of a WeakPtr<Base> to WeakPtr<Derived>. This 169 // conversion will only compile if there is exists a Base which inherits 170 // from SupportsWeakPtr<Base>. See base::AsWeakPtr() below for a helper 171 // function that makes calling this easier. 172 template<typename Derived> 173 static WeakPtr<Derived> StaticAsWeakPtr(Derived* t) { 174 static_assert( 175 std::is_base_of<internal::SupportsWeakPtrBase, Derived>::value, 176 "AsWeakPtr argument must inherit from SupportsWeakPtr"); 177 return AsWeakPtrImpl<Derived>(t, *t); 178 } 179 180 private: 181 // This template function uses type inference to find a Base of Derived 182 // which is an instance of SupportsWeakPtr<Base>. We can then safely 183 // static_cast the Base* to a Derived*. 184 template <typename Derived, typename Base> 185 static WeakPtr<Derived> AsWeakPtrImpl( 186 Derived* t, const SupportsWeakPtr<Base>&) { 187 WeakPtr<Base> ptr = t->Base::AsWeakPtr(); 188 return WeakPtr<Derived>(ptr.ref_, static_cast<Derived*>(ptr.ptr_)); 189 } 190}; 191 192} // namespace internal 193 194template <typename T> class WeakPtrFactory; 195 196// The WeakPtr class holds a weak reference to |T*|. 197// 198// This class is designed to be used like a normal pointer. You should always 199// null-test an object of this class before using it or invoking a method that 200// may result in the underlying object being destroyed. 201// 202// EXAMPLE: 203// 204// class Foo { ... }; 205// WeakPtr<Foo> foo; 206// if (foo) 207// foo->method(); 208// 209template <typename T> 210class WeakPtr : public internal::WeakPtrBase { 211 public: 212 WeakPtr() : ptr_(nullptr) {} 213 214 WeakPtr(std::nullptr_t) : ptr_(nullptr) {} 215 216 // Allow conversion from U to T provided U "is a" T. Note that this 217 // is separate from the (implicit) copy and move constructors. 218 template <typename U> 219 WeakPtr(const WeakPtr<U>& other) : WeakPtrBase(other), ptr_(other.ptr_) { 220 } 221 template <typename U> 222 WeakPtr(WeakPtr<U>&& other) 223 : WeakPtrBase(std::move(other)), ptr_(other.ptr_) {} 224 225 T* get() const { return ref_.is_valid() ? ptr_ : nullptr; } 226 227 T& operator*() const { 228 DCHECK(get() != nullptr); 229 return *get(); 230 } 231 T* operator->() const { 232 DCHECK(get() != nullptr); 233 return get(); 234 } 235 236 void reset() { 237 ref_ = internal::WeakReference(); 238 ptr_ = nullptr; 239 } 240 241 // Allow conditionals to test validity, e.g. if (weak_ptr) {...}; 242 explicit operator bool() const { return get() != nullptr; } 243 244 private: 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 nullptr). 257 T* ptr_; 258}; 259 260// Allow callers to compare WeakPtrs against nullptr to test validity. 261template <class T> 262bool operator!=(const WeakPtr<T>& weak_ptr, std::nullptr_t) { 263 return !(weak_ptr == nullptr); 264} 265template <class T> 266bool operator!=(std::nullptr_t, const WeakPtr<T>& weak_ptr) { 267 return weak_ptr != nullptr; 268} 269template <class T> 270bool operator==(const WeakPtr<T>& weak_ptr, std::nullptr_t) { 271 return weak_ptr.get() == nullptr; 272} 273template <class T> 274bool operator==(std::nullptr_t, const WeakPtr<T>& weak_ptr) { 275 return weak_ptr == nullptr; 276} 277 278// A class may be composed of a WeakPtrFactory and thereby 279// control how it exposes weak pointers to itself. This is helpful if you only 280// need weak pointers within the implementation of a class. This class is also 281// useful when working with primitive types. For example, you could have a 282// WeakPtrFactory<bool> that is used to pass around a weak reference to a bool. 283template <class T> 284class WeakPtrFactory { 285 public: 286 explicit WeakPtrFactory(T* ptr) : ptr_(ptr) { 287 } 288 289 ~WeakPtrFactory() { ptr_ = nullptr; } 290 291 WeakPtr<T> GetWeakPtr() { 292 DCHECK(ptr_); 293 return WeakPtr<T>(weak_reference_owner_.GetRef(), ptr_); 294 } 295 296 // Call this method to invalidate all existing weak pointers. 297 void InvalidateWeakPtrs() { 298 DCHECK(ptr_); 299 weak_reference_owner_.Invalidate(); 300 } 301 302 // Call this method to determine if any weak pointers exist. 303 bool HasWeakPtrs() const { 304 DCHECK(ptr_); 305 return weak_reference_owner_.HasRefs(); 306 } 307 308 private: 309 internal::WeakReferenceOwner weak_reference_owner_; 310 T* ptr_; 311 DISALLOW_IMPLICIT_CONSTRUCTORS(WeakPtrFactory); 312}; 313 314// A class may extend from SupportsWeakPtr to let others take weak pointers to 315// it. This avoids the class itself implementing boilerplate to dispense weak 316// pointers. However, since SupportsWeakPtr's destructor won't invalidate 317// weak pointers to the class until after the derived class' members have been 318// destroyed, its use can lead to subtle use-after-destroy issues. 319template <class T> 320class SupportsWeakPtr : public internal::SupportsWeakPtrBase { 321 public: 322 SupportsWeakPtr() {} 323 324 WeakPtr<T> AsWeakPtr() { 325 return WeakPtr<T>(weak_reference_owner_.GetRef(), static_cast<T*>(this)); 326 } 327 328 protected: 329 ~SupportsWeakPtr() {} 330 331 private: 332 internal::WeakReferenceOwner weak_reference_owner_; 333 DISALLOW_COPY_AND_ASSIGN(SupportsWeakPtr); 334}; 335 336// Helper function that uses type deduction to safely return a WeakPtr<Derived> 337// when Derived doesn't directly extend SupportsWeakPtr<Derived>, instead it 338// extends a Base that extends SupportsWeakPtr<Base>. 339// 340// EXAMPLE: 341// class Base : public base::SupportsWeakPtr<Producer> {}; 342// class Derived : public Base {}; 343// 344// Derived derived; 345// base::WeakPtr<Derived> ptr = base::AsWeakPtr(&derived); 346// 347// Note that the following doesn't work (invalid type conversion) since 348// Derived::AsWeakPtr() is WeakPtr<Base> SupportsWeakPtr<Base>::AsWeakPtr(), 349// and there's no way to safely cast WeakPtr<Base> to WeakPtr<Derived> at 350// the caller. 351// 352// base::WeakPtr<Derived> ptr = derived.AsWeakPtr(); // Fails. 353 354template <typename Derived> 355WeakPtr<Derived> AsWeakPtr(Derived* t) { 356 return internal::SupportsWeakPtrBase::StaticAsWeakPtr<Derived>(t); 357} 358 359} // namespace base 360 361#endif // BASE_MEMORY_WEAK_PTR_H_ 362