Add support for WeakPtr<T>

webrtc/base/weak_ptr.h

+/*
+ *  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 WEBRTC_BASE_WEAK_PTR_H_
+#define WEBRTC_BASE_WEAK_PTR_H_
+
+#include <memory>
+
+#include <utility>
+
+#include "webrtc/base/refcount.h"
+#include "webrtc/base/scoped_ref_ptr.h"
+#include "webrtc/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.
+
+// 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;
+
+    SequencedTaskChecker 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:
+  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 {
+ 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() { 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_;
+  RTC_DISALLOW_IMPLICIT_CONSTRUCTORS(WeakPtrFactory);
+};
+
+}  // namespace rtc
+
+#endif  // WEBRTC_BASE_WEAK_PTR_H_