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 82 namespace base { 83 84 template <typename T> class SupportsWeakPtr; 85 template <typename T> class WeakPtr; 86 87 namespace internal { 88 // These classes are part of the WeakPtr implementation. 89 // DO NOT USE THESE CLASSES DIRECTLY YOURSELF. 90 91 class 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 }; 125 126 class 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. 147 class 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<>. 166 class 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 193 194 template <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 // 209 template <typename T> 210 class 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. 261 template <class T> 262 bool operator!=(const WeakPtr<T>& weak_ptr, std::nullptr_t) { 263 return !(weak_ptr == nullptr); 264 } 265 template <class T> 266 bool operator!=(std::nullptr_t, const WeakPtr<T>& weak_ptr) { 267 return weak_ptr != nullptr; 268 } 269 template <class T> 270 bool operator==(const WeakPtr<T>& weak_ptr, std::nullptr_t) { 271 return weak_ptr.get() == nullptr; 272 } 273 template <class T> 274 bool 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. 283 template <class T> 284 class 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. 319 template <class T> 320 class 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. 353 354 template <typename Derived> 355 WeakPtr<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