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- /*
- * Copyright 2016 The WebRTC Project Authors. All rights reserved.
- *
- * Use of this source code is governed by a BSD-style license
- * that can be found in the LICENSE file in the root of the source
- * tree. An additional intellectual property rights grant can be found
- * in the file PATENTS. All contributing project authors may
- * be found in the AUTHORS file in the root of the source tree.
- */
- #ifndef RTC_BASE_WEAK_PTR_H_
- #define RTC_BASE_WEAK_PTR_H_
- #include <memory>
- #include <utility>
- #include "api/scoped_refptr.h"
- #include "rtc_base/ref_count.h"
- #include "rtc_base/ref_counted_object.h"
- #include "rtc_base/synchronization/sequence_checker.h"
- #include "rtc_base/system/no_unique_address.h"
- // The implementation is borrowed from chromium except that it does not
- // implement SupportsWeakPtr.
- // 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 TaskQueue or thread, 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
- // TaskQueue/thread, and cannot be dereferenced or
- // invalidated on any other TaskQueue/thread. Bound WeakPtrs can still be handed
- // off to other TaskQueues, e.g. to use to post tasks back to object on the
- // bound 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.
- namespace rtc {
- namespace internal {
- class WeakReference {
- public:
- // Although Flag is bound to a specific sequence, it may be
- // deleted from another via base::WeakPtr::~WeakPtr().
- class Flag : public RefCountInterface {
- public:
- Flag();
- void Invalidate();
- bool IsValid() const;
- private:
- friend class RefCountedObject<Flag>;
- ~Flag() override;
- RTC_NO_UNIQUE_ADDRESS ::webrtc::SequenceChecker checker_;
- 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 WeakReferenceOwner {
- public:
- WeakReferenceOwner();
- ~WeakReferenceOwner();
- WeakReference GetRef() const;
- bool HasRefs() const { return flag_.get() && !flag_->HasOneRef(); }
- void Invalidate();
- private:
- mutable scoped_refptr<RefCountedObject<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 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_;
- };
- } // namespace internal
- template <typename T>
- class WeakPtrFactory;
- template <typename T>
- class WeakPtr : public internal::WeakPtrBase {
- public:
- WeakPtr() : 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)
- : internal::WeakPtrBase(other), ptr_(other.ptr_) {}
- template <typename U>
- WeakPtr(WeakPtr<U>&& other)
- : internal::WeakPtrBase(std::move(other)), ptr_(other.ptr_) {}
- T* get() const { return ref_.is_valid() ? ptr_ : nullptr; }
- T& operator*() const {
- RTC_DCHECK(get() != nullptr);
- return *get();
- }
- T* operator->() const {
- RTC_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:
- template <typename U>
- friend class WeakPtr;
- friend class WeakPtrFactory<T>;
- WeakPtr(const internal::WeakReference& ref, T* ptr)
- : internal::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.
- // Note that GetWeakPtr must be called on one and only one TaskQueue or thread
- // and the WeakPtr must only be dereferenced and invalidated on that same
- // TaskQueue/thread. A WeakPtr instance can be copied and posted to other
- // sequences though as long as it is not dereferenced (WeakPtr<T>::get()).
- template <class T>
- class WeakPtrFactory {
- public:
- explicit WeakPtrFactory(T* ptr) : ptr_(ptr) {}
- WeakPtrFactory() = delete;
- WeakPtrFactory(const WeakPtrFactory&) = delete;
- WeakPtrFactory& operator=(const WeakPtrFactory&) = delete;
- ~WeakPtrFactory() { ptr_ = nullptr; }
- WeakPtr<T> GetWeakPtr() {
- RTC_DCHECK(ptr_);
- return WeakPtr<T>(weak_reference_owner_.GetRef(), ptr_);
- }
- // Call this method to invalidate all existing weak pointers.
- void InvalidateWeakPtrs() {
- RTC_DCHECK(ptr_);
- weak_reference_owner_.Invalidate();
- }
- // Call this method to determine if any weak pointers exist.
- bool HasWeakPtrs() const {
- RTC_DCHECK(ptr_);
- return weak_reference_owner_.HasRefs();
- }
- private:
- internal::WeakReferenceOwner weak_reference_owner_;
- T* ptr_;
- };
- } // namespace rtc
- #endif // RTC_BASE_WEAK_PTR_H_
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