Add a WeakPtr<T> class....

base/weak_ptr.h

+// Copyright (c) 2009 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 help in cases where you have many objects referring back to a
+// shared object and you wish for the lifetime of the shared object to not be
+// bound to the lifetime of the referrers.  In other words, this is useful when
+// reference counting is not a good fit.
+//
+// A common alternative to weak pointers is to have the shared object hold a
+// list of all referrers, and then when the shared object is destroyed, it
+// calls a method on the referrers to tell them to drop their references.  This
+// approach also requires the referrers to tell the shared object when they get
+// destroyed so that the shared object can remove the referrer from its list of
+// referrers.  Such a solution works, but it is a bit complex.
+//
+// EXAMPLE:
+//
+//  class Controller : public SupportsWeakPtr {
+//   public:
+//    void SpawnWorker() { Worker::StartNew(GetWeakPtr()); }
+//    void WorkComplete(const Result& result) { ... }
+//  };
+//
+//  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_;
+//  };
+//
+// Given the above classes, a consumer may allocate a Controller object, call
+// SpawnWorker several times, and then destroy the Controller object before all
+// of the workers have completed.  Because the Worker class only holds a weak
+// pointer to the Controller, we don't have to worry about the Worker
+// dereferencing the Controller back pointer after the Controller has been
+// destroyed.
+//
+// WARNING: weak pointers are not threadsafe!!!  You must only use a WeakPtr
+// instance on thread where it was created.
+
+#ifndef BASE_WEAK_PTR_H_
+#define BASE_WEAK_PTR_H_
+
+#include "base/logging.h"
+#include "base/non_thread_safe.h"
+#include "base/ref_counted.h"
+
+namespace base {
+
+namespace internal {
+// These classes are part of the WeakPtr implementation.
+// DO NOT USE THESE CLASSES DIRECTLY YOURSELF.
+
+class WeakReference {
+ public:
+  void EnsureInitialized() {
+    // Lazy initialization helps faciliate the NonThreadSafe debug checks.
+    if (!flag_) {
+      flag_ = new Flag();
+      flag_->data = true;
+    }
+  }
+
+  void Invalidate() {
+    if (flag_)
+      flag_->data = false;
+  }
+
+  bool is_valid() const { return flag_ && flag_->data; }
+
+ private:
+  // A reference counted boolean that is true when the weak reference is valid
+  // and false otherwise.
+  class Flag : public RefCountedData<bool>, public NonThreadSafe {
+  };
+
+  scoped_refptr<Flag> flag_;
+};
+
+class WeakReferenceOwner {
+ public:
+  ~WeakReferenceOwner() {
+    ref_.Invalidate();
+  }
+
+  const WeakReference& GetRef() const {
+    ref_.EnsureInitialized();
+    return ref_;
+  }
+
+  void Invalidate() { ref_.Invalidate(); }
+
+ private:
+  mutable WeakReference ref_;
+};
+
+// 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() {
+  }
+
+ protected:
+  WeakPtrBase(const WeakReference& ref) : ref_(ref) {
+  }
+
+  WeakReference ref_;
+};
+
+}  // namespace internal
+
+template <typename T> class SupportsWeakPtr;
+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_(NULL) {
+  }
+
+  // Allow conversion from U to T provided U "is a" T.
+  template <typename U>
+  WeakPtr(const WeakPtr<U>& other) : WeakPtrBase(other), ptr_(other.get()) {
+  }
+
+  T* get() const { return ref_.is_valid() ? ptr_ : NULL; }
+  operator T*() const { return get(); }
+
+  T* operator*() const {
+    DCHECK(get() != NULL);
+    return *get();
+  }
+  T* operator->() const {
+    DCHECK(get() != NULL);
+    return get();
+  }
+
+ private:
+  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 NULL).
+  T* ptr_;
+};
+
+// A class may extend from SupportsWeakPtr to expose weak pointers to itself.
+// This is useful in cases where you want others to be able to get a weak
+// pointer to your class.  It also has the property that you don't need to
+// initialize it from your constructor.
+template <class T>
+class SupportsWeakPtr {
+ public:
+  SupportsWeakPtr() {}
+
+  WeakPtr<T> AsWeakPtr() {
+    return WeakPtr<T>(weak_reference_owner_.GetRef(), static_cast<T*>(this));
+  }
+
+ private:
+  internal::WeakReferenceOwner weak_reference_owner_;
+  DISALLOW_COPY_AND_ASSIGN(SupportsWeakPtr);
+};
+
+// A class may alternatively 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) {
+  }
+
+  WeakPtr<T> GetWeakPtr() {
+    return WeakPtr<T>(weak_reference_owner_.GetRef(), ptr_);
+  }
+
+  // Call this method to invalidate all existing weak pointers.
+  void InvalidateWeakPtrs() { weak_reference_owner_.Invalidate(); }
+
+ private:
+  internal::WeakReferenceOwner weak_reference_owner_;
+  T* ptr_;
+  DISALLOW_IMPLICIT_CONSTRUCTORS(WeakPtrFactory);
+};
+
+}  // namespace base
+
+#endif  // BASE_WEAK_PTR_H_