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diff --git a/mfbt/WeakPtr.h b/mfbt/WeakPtr.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Weak pointer functionality, implemented as a mixin for use with any class. */
+
+/**
+ * SupportsWeakPtr lets you have a pointer to an object 'Foo' without affecting
+ * its lifetime. It works by creating a single shared reference counted object
+ * (WeakReference) that each WeakPtr will access 'Foo' through. This lets 'Foo'
+ * clear the pointer in the WeakReference without having to know about all of
+ * the WeakPtrs to it and allows the WeakReference to live beyond the lifetime
+ * of 'Foo'.
+ *
+ * The overhead of WeakPtr is that accesses to 'Foo' becomes an additional
+ * dereference, and an additional heap allocated pointer sized object shared
+ * between all of the WeakPtrs.
+ *
+ * Example of usage:
+ *
+ *   // To have a class C support weak pointers, inherit from SupportsWeakPtr<C>.
+ *   class C : public SupportsWeakPtr<C>
+ *   {
+ *    public:
+ *      int num;
+ *      void act();
+ *   };
+ *
+ *   C* ptr =  new C();
+ *
+ *   // Get weak pointers to ptr. The first time asWeakPtr is called
+ *   // a reference counted WeakReference object is created that
+ *   // can live beyond the lifetime of 'ptr'. The WeakReference
+ *   // object will be notified of 'ptr's destruction.
+ *   WeakPtr<C> weak = ptr->asWeakPtr();
+ *   WeakPtr<C> other = ptr->asWeakPtr();
+ *
+ *   // Test a weak pointer for validity before using it.
+ *   if (weak) {
+ *     weak->num = 17;
+ *     weak->act();
+ *   }
+ *
+ *   // Destroying the underlying object clears weak pointers to it.
+ *   delete ptr;
+ *
+ *   MOZ_ASSERT(!weak, "Deleting |ptr| clears weak pointers to it.");
+ *   MOZ_ASSERT(!other, "Deleting |ptr| clears all weak pointers to it.");
+ *
+ * WeakPtr is typesafe and may be used with any class. It is not required that
+ * the class be reference-counted or allocated in any particular way.
+ *
+ * The API was loosely inspired by Chromium's weak_ptr.h:
+ * http://src.chromium.org/svn/trunk/src/base/memory/weak_ptr.h
+ */
+
+#ifndef mozilla_WeakPtr_h_
+#define mozilla_WeakPtr_h_
+
+#include "mozilla/Assertions.h"
+#include "mozilla/NullPtr.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/TypeTraits.h"
+
+namespace mozilla {
+
+template <typename T> class WeakPtr;
+
+template <typename T>
+class SupportsWeakPtr
+{
+  public:
+    WeakPtr<T> asWeakPtr() {
+      if (!weakRef)
+        weakRef = new WeakReference(static_cast<T*>(this));
+      return WeakPtr<T>(weakRef);
+    }
+
+  protected:
+    ~SupportsWeakPtr() {
+      MOZ_STATIC_ASSERT((IsBaseOf<SupportsWeakPtr<T>, T>::value), "T must derive from SupportsWeakPtr<T>");
+      if (weakRef)
+        weakRef->detach();
+    }
+
+  private:
+    friend class WeakPtr<T>;
+
+    // This can live beyond the lifetime of the class derived from SupportsWeakPtr.
+    class WeakReference : public RefCounted<WeakReference>
+    {
+      public:
+        explicit WeakReference(T* ptr) : ptr(ptr) {}
+        T* get() const {
+          return ptr;
+        }
+
+      private:
+        friend class WeakPtr<T>;
+        friend class SupportsWeakPtr<T>;
+        void detach() {
+          ptr = nullptr;
+        }
+        T* ptr;
+    };
+
+    RefPtr<WeakReference> weakRef;
+};
+
+template <typename T>
+class WeakPtr
+{
+  public:
+    WeakPtr(const WeakPtr<T>& o) : ref(o.ref) {}
+    WeakPtr() : ref(nullptr) {}
+
+    operator T*() const {
+      return ref->get();
+    }
+    T& operator*() const {
+      return *ref->get();
+    }
+
+    T* operator->() const {
+      return ref->get();
+    }
+
+  private:
+    friend class SupportsWeakPtr<T>;
+
+    explicit WeakPtr(const RefPtr<typename SupportsWeakPtr<T>::WeakReference> &o) : ref(o) {}
+
+    RefPtr<typename SupportsWeakPtr<T>::WeakReference> ref;
+};
+
+} // namespace mozilla
+
+#endif /* ifdef mozilla_WeakPtr_h_ */