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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.
#include <memory>
#include <utility>
#include "webrtc/rtc_base/refcount.h"
#include "webrtc/rtc_base/scoped_ref_ptr.h"
#include "webrtc/rtc_base/sequenced_task_checker.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.
// 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 {
// Although Flag is bound to a specific sequence, it may be
// deleted from another via base::WeakPtr::~WeakPtr().
class Flag : public RefCountInterface {
void Invalidate();
bool IsValid() const;
friend class RefCountedObject<Flag>;
~Flag() override;
SequencedTaskChecker checker_;
bool is_valid_;
explicit WeakReference(const Flag* flag);
WeakReference(WeakReference&& other);
WeakReference(const WeakReference& other);
WeakReference& operator=(WeakReference&& other) = default;
WeakReference& operator=(const WeakReference& other) = default;
bool is_valid() const;
scoped_refptr<const Flag> flag_;
class WeakReferenceOwner {
WeakReference GetRef() const;
bool HasRefs() const { return flag_.get() && !flag_->HasOneRef(); }
void Invalidate();
SequencedTaskChecker checker_;
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 {
WeakPtrBase(const WeakPtrBase& other) = default;
WeakPtrBase(WeakPtrBase&& other) = default;
WeakPtrBase& operator=(const WeakPtrBase& other) = default;
WeakPtrBase& operator=(WeakPtrBase&& other) = default;
explicit WeakPtrBase(const WeakReference& ref);
WeakReference ref_;
} // namespace internal
template <typename T>
class WeakPtrFactory;
template <typename T>
class WeakPtr : public internal::WeakPtrBase {
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; }
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 {
explicit WeakPtrFactory(T* ptr) : ptr_(ptr) {}
~WeakPtrFactory() { ptr_ = nullptr; }
WeakPtr<T> GetWeakPtr() {
return WeakPtr<T>(weak_reference_owner_.GetRef(), ptr_);
// Call this method to invalidate all existing weak pointers.
void InvalidateWeakPtrs() {
// Call this method to determine if any weak pointers exist.
bool HasWeakPtrs() const {
return weak_reference_owner_.HasRefs();
internal::WeakReferenceOwner weak_reference_owner_;
T* ptr_;
} // namespace rtc