| // Copyright (c) 2012 The Chromium Authors. All rights reserved. |
| // Use of this source code is governed by a BSD-style license that can be |
| // found in the LICENSE file. |
| |
| // Weak pointers are pointers to an object that do not affect its lifetime, |
| // and which may be invalidated (i.e. reset to nullptr) by the object, or its |
| // owner, at any time, most commonly when the object is about to be deleted. |
| |
| // Weak pointers are useful when an object needs to be accessed safely by one |
| // or more objects other than its owner, and those callers can cope with the |
| // object vanishing and e.g. tasks posted to it being silently dropped. |
| // Reference-counting such an object would complicate the ownership graph and |
| // make it harder to reason about the object's lifetime. |
| |
| // EXAMPLE: |
| // |
| // class Controller { |
| // public: |
| // Controller() : weak_factory_(this) {} |
| // void SpawnWorker() { Worker::StartNew(weak_factory_.GetWeakPtr()); } |
| // void WorkComplete(const Result& result) { ... } |
| // private: |
| // // Member variables should appear before the WeakPtrFactory, to ensure |
| // // that any WeakPtrs to Controller are invalidated before its members |
| // // variable's destructors are executed, rendering them invalid. |
| // WeakPtrFactory<Controller> weak_factory_; |
| // }; |
| // |
| // class Worker { |
| // public: |
| // static void StartNew(const WeakPtr<Controller>& controller) { |
| // Worker* worker = new Worker(controller); |
| // // Kick off asynchronous processing... |
| // } |
| // private: |
| // Worker(const WeakPtr<Controller>& controller) |
| // : controller_(controller) {} |
| // void DidCompleteAsynchronousProcessing(const Result& result) { |
| // if (controller_) |
| // controller_->WorkComplete(result); |
| // } |
| // WeakPtr<Controller> controller_; |
| // }; |
| // |
| // With this implementation a caller may use SpawnWorker() to dispatch multiple |
| // Workers and subsequently delete the Controller, without waiting for all |
| // Workers to have completed. |
| |
| // ------------------------- IMPORTANT: Thread-safety ------------------------- |
| |
| // Weak pointers may be passed safely between threads, but must always be |
| // dereferenced and invalidated on the same SequencedTaskRunner otherwise |
| // checking the pointer would be racey. |
| // |
| // To ensure correct use, the first time a WeakPtr issued by a WeakPtrFactory |
| // is dereferenced, the factory and its WeakPtrs become bound to the calling |
| // thread or current SequencedWorkerPool token, and cannot be dereferenced or |
| // invalidated on any other task runner. Bound WeakPtrs can still be handed |
| // off to other task runners, e.g. to use to post tasks back to object on the |
| // bound sequence. |
| // |
| // If all WeakPtr objects are destroyed or invalidated then the factory is |
| // unbound from the SequencedTaskRunner/Thread. The WeakPtrFactory may then be |
| // destroyed, or new WeakPtr objects may be used, from a different sequence. |
| // |
| // Thus, at least one WeakPtr object must exist and have been dereferenced on |
| // the correct thread to enforce that other WeakPtr objects will enforce they |
| // are used on the desired thread. |
| |
| #ifndef BASE_MEMORY_WEAK_PTR_H_ |
| #define BASE_MEMORY_WEAK_PTR_H_ |
| |
| #include <cstddef> |
| #include <type_traits> |
| |
| #include "base/base_export.h" |
| #include "base/logging.h" |
| #include "base/macros.h" |
| #include "base/memory/ref_counted.h" |
| |
| namespace base { |
| |
| template <typename T> class SupportsWeakPtr; |
| template <typename T> class WeakPtr; |
| |
| namespace internal { |
| // These classes are part of the WeakPtr implementation. |
| // DO NOT USE THESE CLASSES DIRECTLY YOURSELF. |
| |
| class BASE_EXPORT WeakReference { |
| public: |
| // Although Flag is bound to a specific SequencedTaskRunner, it may be |
| // deleted from another via base::WeakPtr::~WeakPtr(). |
| class Flag : public RefCountedThreadSafe<Flag> { |
| public: |
| Flag(); |
| |
| void Invalidate(); |
| bool IsValid() const; |
| |
| private: |
| friend class base::RefCountedThreadSafe<Flag>; |
| |
| ~Flag(); |
| |
| bool is_valid_; |
| }; |
| |
| WeakReference(); |
| explicit WeakReference(const Flag* flag); |
| ~WeakReference(); |
| |
| WeakReference(WeakReference&& other); |
| WeakReference(const WeakReference& other); |
| WeakReference& operator=(WeakReference&& other) = default; |
| WeakReference& operator=(const WeakReference& other) = default; |
| |
| bool is_valid() const; |
| |
| private: |
| scoped_refptr<const Flag> flag_; |
| }; |
| |
| class BASE_EXPORT WeakReferenceOwner { |
| public: |
| WeakReferenceOwner(); |
| ~WeakReferenceOwner(); |
| |
| WeakReference GetRef() const; |
| |
| bool HasRefs() const { |
| return flag_.get() && !flag_->HasOneRef(); |
| } |
| |
| void Invalidate(); |
| |
| private: |
| mutable scoped_refptr<WeakReference::Flag> flag_; |
| }; |
| |
| // This class simplifies the implementation of WeakPtr's type conversion |
| // constructor by avoiding the need for a public accessor for ref_. A |
| // WeakPtr<T> cannot access the private members of WeakPtr<U>, so this |
| // base class gives us a way to access ref_ in a protected fashion. |
| class BASE_EXPORT WeakPtrBase { |
| public: |
| WeakPtrBase(); |
| ~WeakPtrBase(); |
| |
| WeakPtrBase(const WeakPtrBase& other) = default; |
| WeakPtrBase(WeakPtrBase&& other) = default; |
| WeakPtrBase& operator=(const WeakPtrBase& other) = default; |
| WeakPtrBase& operator=(WeakPtrBase&& other) = default; |
| |
| protected: |
| explicit WeakPtrBase(const WeakReference& ref); |
| |
| WeakReference ref_; |
| }; |
| |
| // This class provides a common implementation of common functions that would |
| // otherwise get instantiated separately for each distinct instantiation of |
| // SupportsWeakPtr<>. |
| class SupportsWeakPtrBase { |
| public: |
| // A safe static downcast of a WeakPtr<Base> to WeakPtr<Derived>. This |
| // conversion will only compile if there is exists a Base which inherits |
| // from SupportsWeakPtr<Base>. See base::AsWeakPtr() below for a helper |
| // function that makes calling this easier. |
| template<typename Derived> |
| static WeakPtr<Derived> StaticAsWeakPtr(Derived* t) { |
| static_assert( |
| std::is_base_of<internal::SupportsWeakPtrBase, Derived>::value, |
| "AsWeakPtr argument must inherit from SupportsWeakPtr"); |
| return AsWeakPtrImpl<Derived>(t, *t); |
| } |
| |
| private: |
| // This template function uses type inference to find a Base of Derived |
| // which is an instance of SupportsWeakPtr<Base>. We can then safely |
| // static_cast the Base* to a Derived*. |
| template <typename Derived, typename Base> |
| static WeakPtr<Derived> AsWeakPtrImpl( |
| Derived* t, const SupportsWeakPtr<Base>&) { |
| WeakPtr<Base> ptr = t->Base::AsWeakPtr(); |
| return WeakPtr<Derived>(ptr.ref_, static_cast<Derived*>(ptr.ptr_)); |
| } |
| }; |
| |
| } // namespace internal |
| |
| template <typename T> class WeakPtrFactory; |
| |
| // The WeakPtr class holds a weak reference to |T*|. |
| // |
| // This class is designed to be used like a normal pointer. You should always |
| // null-test an object of this class before using it or invoking a method that |
| // may result in the underlying object being destroyed. |
| // |
| // EXAMPLE: |
| // |
| // class Foo { ... }; |
| // WeakPtr<Foo> foo; |
| // if (foo) |
| // foo->method(); |
| // |
| template <typename T> |
| class WeakPtr : public internal::WeakPtrBase { |
| public: |
| WeakPtr() : ptr_(nullptr) {} |
| |
| WeakPtr(std::nullptr_t) : ptr_(nullptr) {} |
| |
| // Allow conversion from U to T provided U "is a" T. Note that this |
| // is separate from the (implicit) copy and move constructors. |
| template <typename U> |
| WeakPtr(const WeakPtr<U>& other) : WeakPtrBase(other), ptr_(other.ptr_) { |
| } |
| template <typename U> |
| WeakPtr(WeakPtr<U>&& other) |
| : WeakPtrBase(std::move(other)), ptr_(other.ptr_) {} |
| |
| T* get() const { return ref_.is_valid() ? ptr_ : nullptr; } |
| |
| T& operator*() const { |
| DCHECK(get() != nullptr); |
| return *get(); |
| } |
| T* operator->() const { |
| DCHECK(get() != nullptr); |
| return get(); |
| } |
| |
| void reset() { |
| ref_ = internal::WeakReference(); |
| ptr_ = nullptr; |
| } |
| |
| // Allow conditionals to test validity, e.g. if (weak_ptr) {...}; |
| explicit operator bool() const { return get() != nullptr; } |
| |
| private: |
| friend class internal::SupportsWeakPtrBase; |
| template <typename U> friend class WeakPtr; |
| friend class SupportsWeakPtr<T>; |
| friend class WeakPtrFactory<T>; |
| |
| WeakPtr(const internal::WeakReference& ref, T* ptr) |
| : WeakPtrBase(ref), |
| ptr_(ptr) { |
| } |
| |
| // This pointer is only valid when ref_.is_valid() is true. Otherwise, its |
| // value is undefined (as opposed to nullptr). |
| T* ptr_; |
| }; |
| |
| // Allow callers to compare WeakPtrs against nullptr to test validity. |
| template <class T> |
| bool operator!=(const WeakPtr<T>& weak_ptr, std::nullptr_t) { |
| return !(weak_ptr == nullptr); |
| } |
| template <class T> |
| bool operator!=(std::nullptr_t, const WeakPtr<T>& weak_ptr) { |
| return weak_ptr != nullptr; |
| } |
| template <class T> |
| bool operator==(const WeakPtr<T>& weak_ptr, std::nullptr_t) { |
| return weak_ptr.get() == nullptr; |
| } |
| template <class T> |
| bool operator==(std::nullptr_t, const WeakPtr<T>& weak_ptr) { |
| return weak_ptr == nullptr; |
| } |
| |
| // A class may be composed of a WeakPtrFactory and thereby |
| // control how it exposes weak pointers to itself. This is helpful if you only |
| // need weak pointers within the implementation of a class. This class is also |
| // useful when working with primitive types. For example, you could have a |
| // WeakPtrFactory<bool> that is used to pass around a weak reference to a bool. |
| template <class T> |
| class WeakPtrFactory { |
| public: |
| explicit WeakPtrFactory(T* ptr) : ptr_(ptr) { |
| } |
| |
| ~WeakPtrFactory() { ptr_ = nullptr; } |
| |
| WeakPtr<T> GetWeakPtr() { |
| DCHECK(ptr_); |
| return WeakPtr<T>(weak_reference_owner_.GetRef(), ptr_); |
| } |
| |
| // Call this method to invalidate all existing weak pointers. |
| void InvalidateWeakPtrs() { |
| DCHECK(ptr_); |
| weak_reference_owner_.Invalidate(); |
| } |
| |
| // Call this method to determine if any weak pointers exist. |
| bool HasWeakPtrs() const { |
| DCHECK(ptr_); |
| return weak_reference_owner_.HasRefs(); |
| } |
| |
| private: |
| internal::WeakReferenceOwner weak_reference_owner_; |
| T* ptr_; |
| DISALLOW_IMPLICIT_CONSTRUCTORS(WeakPtrFactory); |
| }; |
| |
| // A class may extend from SupportsWeakPtr to let others take weak pointers to |
| // it. This avoids the class itself implementing boilerplate to dispense weak |
| // pointers. However, since SupportsWeakPtr's destructor won't invalidate |
| // weak pointers to the class until after the derived class' members have been |
| // destroyed, its use can lead to subtle use-after-destroy issues. |
| template <class T> |
| class SupportsWeakPtr : public internal::SupportsWeakPtrBase { |
| public: |
| SupportsWeakPtr() {} |
| |
| WeakPtr<T> AsWeakPtr() { |
| return WeakPtr<T>(weak_reference_owner_.GetRef(), static_cast<T*>(this)); |
| } |
| |
| protected: |
| ~SupportsWeakPtr() {} |
| |
| private: |
| internal::WeakReferenceOwner weak_reference_owner_; |
| DISALLOW_COPY_AND_ASSIGN(SupportsWeakPtr); |
| }; |
| |
| // Helper function that uses type deduction to safely return a WeakPtr<Derived> |
| // when Derived doesn't directly extend SupportsWeakPtr<Derived>, instead it |
| // extends a Base that extends SupportsWeakPtr<Base>. |
| // |
| // EXAMPLE: |
| // class Base : public base::SupportsWeakPtr<Producer> {}; |
| // class Derived : public Base {}; |
| // |
| // Derived derived; |
| // base::WeakPtr<Derived> ptr = base::AsWeakPtr(&derived); |
| // |
| // Note that the following doesn't work (invalid type conversion) since |
| // Derived::AsWeakPtr() is WeakPtr<Base> SupportsWeakPtr<Base>::AsWeakPtr(), |
| // and there's no way to safely cast WeakPtr<Base> to WeakPtr<Derived> at |
| // the caller. |
| // |
| // base::WeakPtr<Derived> ptr = derived.AsWeakPtr(); // Fails. |
| |
| template <typename Derived> |
| WeakPtr<Derived> AsWeakPtr(Derived* t) { |
| return internal::SupportsWeakPtrBase::StaticAsWeakPtr<Derived>(t); |
| } |
| |
| } // namespace base |
| |
| #endif // BASE_MEMORY_WEAK_PTR_H_ |