// Copyright (c) 2014 Marshall A. Greenblatt. Portions copyright (c) 2012 // Google Inc. All rights reserved. // // Redistribution and use in source and binary forms, with or without // modification, are permitted provided that the following conditions are // met: // // * Redistributions of source code must retain the above copyright // notice, this list of conditions and the following disclaimer. // * Redistributions in binary form must reproduce the above // copyright notice, this list of conditions and the following disclaimer // in the documentation and/or other materials provided with the // distribution. // * Neither the name of Google Inc. nor the name Chromium Embedded // Framework nor the names of its contributors may be used to endorse // or promote products derived from this software without specific prior // written permission. // // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. /// /// \file /// Weak pointers are pointers to an object that do not affect its lifetime. /// They 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:
///    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 weak_factory_{this};
///  };
///
///  class Worker {
///   public:
///    static void StartNew(WeakPtr controller) {
///      Worker* worker = new Worker(std::move(controller));
///      // Kick off asynchronous processing...
///    }
///   private:
///    Worker(WeakPtr controller)
///        : controller_(std::move(controller)) {}
///    void DidCompleteAsynchronousProcessing(const Result& result) {
///      if (controller_)
///        controller_->WorkComplete(result);
///    }
///    WeakPtr 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 ThreaddTaskRunner 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 ThreaddWorkerPool 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 thread. /// /// If all WeakPtr objects are destroyed or invalidated then the factory is /// unbound from the ThreadedTaskRunner/Thread. The WeakPtrFactory may then be /// destroyed, or new WeakPtr objects may be used, from a different thread. /// /// 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 CEF_INCLUDE_BASE_CEF_WEAK_PTR_H_ #define CEF_INCLUDE_BASE_CEF_WEAK_PTR_H_ #pragma once #if defined(USING_CHROMIUM_INCLUDES) // When building CEF include the Chromium header directly. #include "base/memory/weak_ptr.h" #else // !USING_CHROMIUM_INCLUDES // The following is substantially similar to the Chromium implementation. // If the Chromium implementation diverges the below implementation should be // updated to match. #include #include #include "include/base/cef_atomic_flag.h" #include "include/base/cef_logging.h" #include "include/base/cef_ref_counted.h" #include "include/base/cef_thread_checker.h" namespace base { template class SupportsWeakPtr; template class WeakPtr; namespace internal { // These classes are part of the WeakPtr implementation. // DO NOT USE THESE CLASSES DIRECTLY YOURSELF. class WeakReference { public: // Although Flag is bound to a specific ThreaddTaskRunner, it may be // deleted from another via base::WeakPtr::~WeakPtr(). class Flag : public RefCountedThreadSafe { public: Flag(); void Invalidate(); bool IsValid() const; bool MaybeValid() const; void DetachFromThread(); private: friend class base::RefCountedThreadSafe; ~Flag(); base::ThreadChecker thread_checker_; AtomicFlag invalidated_; }; WeakReference(); explicit WeakReference(const scoped_refptr& flag); ~WeakReference(); WeakReference(WeakReference&& other) noexcept; WeakReference(const WeakReference& other); WeakReference& operator=(WeakReference&& other) noexcept = default; WeakReference& operator=(const WeakReference& other) = default; bool IsValid() const; bool MaybeValid() const; private: scoped_refptr flag_; }; class WeakReferenceOwner { public: WeakReferenceOwner(); ~WeakReferenceOwner(); WeakReference GetRef() const; bool HasRefs() const { return !flag_->HasOneRef(); } void Invalidate(); private: scoped_refptr flag_; }; // This class simplifies the implementation of WeakPtr's type conversion // constructor by avoiding the need for a public accessor for ref_. A // WeakPtr cannot access the private members of WeakPtr, so this // base class gives us a way to access ref_ in a protected fashion. class WeakPtrBase { public: WeakPtrBase(); ~WeakPtrBase(); WeakPtrBase(const WeakPtrBase& other) = default; WeakPtrBase(WeakPtrBase&& other) noexcept = default; WeakPtrBase& operator=(const WeakPtrBase& other) = default; WeakPtrBase& operator=(WeakPtrBase&& other) noexcept = default; void reset() { ref_ = internal::WeakReference(); ptr_ = 0; } protected: WeakPtrBase(const WeakReference& ref, uintptr_t ptr); WeakReference ref_; // This pointer is only valid when ref_.is_valid() is true. Otherwise, its // value is undefined (as opposed to nullptr). uintptr_t ptr_; }; // 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 to WeakPtr. This // conversion will only compile if there is exists a Base which inherits // from SupportsWeakPtr. See base::AsWeakPtr() below for a helper // function that makes calling this easier. // // Precondition: t != nullptr template static WeakPtr StaticAsWeakPtr(Derived* t) { static_assert( std::is_base_of::value, "AsWeakPtr argument must inherit from SupportsWeakPtr"); return AsWeakPtrImpl(t); } private: // This template function uses type inference to find a Base of Derived // which is an instance of SupportsWeakPtr. We can then safely // static_cast the Base* to a Derived*. template static WeakPtr AsWeakPtrImpl(SupportsWeakPtr* t) { WeakPtr ptr = t->AsWeakPtr(); return WeakPtr( ptr.ref_, static_cast(reinterpret_cast(ptr.ptr_))); } }; } // namespace internal template 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;
///   if (foo)
///     foo->method();
/// 
/// template class WeakPtr : public internal::WeakPtrBase { public: WeakPtr() = default; WeakPtr(std::nullptr_t) {} /// /// Allow conversion from U to T provided U "is a" T. Note that this /// is separate from the (implicit) copy and move constructors. /// template WeakPtr(const WeakPtr& other) : WeakPtrBase(other) { // Need to cast from U* to T* to do pointer adjustment in case of multiple // inheritance. This also enforces the "U is a T" rule. T* t = reinterpret_cast(other.ptr_); ptr_ = reinterpret_cast(t); } template WeakPtr(WeakPtr&& other) noexcept : WeakPtrBase(std::move(other)) { // Need to cast from U* to T* to do pointer adjustment in case of multiple // inheritance. This also enforces the "U is a T" rule. T* t = reinterpret_cast(other.ptr_); ptr_ = reinterpret_cast(t); } T* get() const { return ref_.IsValid() ? reinterpret_cast(ptr_) : nullptr; } T& operator*() const { CHECK(ref_.IsValid()); return *get(); } T* operator->() const { CHECK(ref_.IsValid()); return get(); } /// /// Allow conditionals to test validity, e.g. `if (weak_ptr) {...}`; /// explicit operator bool() const { return get() != nullptr; } /// /// Returns false if the WeakPtr is confirmed to be invalid. This call is safe /// to make from any thread, e.g. to optimize away unnecessary work, but /// operator bool() must always be called, on the correct thread, before /// actually using the pointer. /// /// Warning: as with any object, this call is only thread-safe if the WeakPtr /// instance isn't being re-assigned or reset() racily with this call. /// bool MaybeValid() const { return ref_.MaybeValid(); } /// /// Returns whether the object |this| points to has been invalidated. This can /// be used to distinguish a WeakPtr to a destroyed object from one that has /// been explicitly set to null. /// bool WasInvalidated() const { return ptr_ && !ref_.IsValid(); } private: friend class internal::SupportsWeakPtrBase; template friend class WeakPtr; friend class SupportsWeakPtr; friend class WeakPtrFactory; WeakPtr(const internal::WeakReference& ref, T* ptr) : WeakPtrBase(ref, reinterpret_cast(ptr)) {} }; /// /// Allow callers to compare WeakPtrs against nullptr to test validity. /// template bool operator!=(const WeakPtr& weak_ptr, std::nullptr_t) { return !(weak_ptr == nullptr); } template bool operator!=(std::nullptr_t, const WeakPtr& weak_ptr) { return weak_ptr != nullptr; } template bool operator==(const WeakPtr& weak_ptr, std::nullptr_t) { return weak_ptr.get() == nullptr; } template bool operator==(std::nullptr_t, const WeakPtr& weak_ptr) { return weak_ptr == nullptr; } namespace internal { class WeakPtrFactoryBase { protected: WeakPtrFactoryBase(uintptr_t ptr); ~WeakPtrFactoryBase(); internal::WeakReferenceOwner weak_reference_owner_; uintptr_t ptr_; }; } // namespace internal /// /// 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 that is used to pass around a weak reference to a /// bool. /// template class WeakPtrFactory : public internal::WeakPtrFactoryBase { public: WeakPtrFactory() = delete; explicit WeakPtrFactory(T* ptr) : WeakPtrFactoryBase(reinterpret_cast(ptr)) {} WeakPtrFactory(const WeakPtrFactory&) = delete; WeakPtrFactory& operator=(const WeakPtrFactory&) = delete; ~WeakPtrFactory() = default; WeakPtr GetWeakPtr() const { return WeakPtr(weak_reference_owner_.GetRef(), reinterpret_cast(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(); } }; /// /// 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 SupportsWeakPtr : public internal::SupportsWeakPtrBase { public: SupportsWeakPtr() = default; SupportsWeakPtr(const SupportsWeakPtr&) = delete; SupportsWeakPtr& operator=(const SupportsWeakPtr&) = delete; WeakPtr AsWeakPtr() { return WeakPtr(weak_reference_owner_.GetRef(), static_cast(this)); } protected: ~SupportsWeakPtr() = default; private: internal::WeakReferenceOwner weak_reference_owner_; }; /// /// Helper function that uses type deduction to safely return a WeakPtr /// when Derived doesn't directly extend SupportsWeakPtr, instead it /// extends a Base that extends SupportsWeakPtr. /// /// EXAMPLE: ///
///   class Base : public base::SupportsWeakPtr {};
///   class Derived : public Base {};
///
///   Derived derived;
///   base::WeakPtr ptr = base::AsWeakPtr(&derived);
/// 
/// /// Note that the following doesn't work (invalid type conversion) since /// Derived::AsWeakPtr() is WeakPtr SupportsWeakPtr::AsWeakPtr(), /// and there's no way to safely cast WeakPtr to WeakPtr at /// the caller. /// ///
///   base::WeakPtr ptr = derived.AsWeakPtr();  // Fails.
/// 
/// template WeakPtr AsWeakPtr(Derived* t) { return internal::SupportsWeakPtrBase::StaticAsWeakPtr(t); } } // namespace base #endif // !USING_CHROMIUM_INCLUDES #endif // CEF_INCLUDE_BASE_CEF_WEAK_PTR_H_