diff -r 000000000000 -r 6474c204b198 toolkit/components/places/nsMaybeWeakPtr.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/toolkit/components/places/nsMaybeWeakPtr.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,105 @@
+/* -*- Mode: C++; tab-width: 8; 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/. */
+
+#ifndef nsMaybeWeakPtr_h_
+#define nsMaybeWeakPtr_h_
+
+#include "nsCOMPtr.h"
+#include "nsWeakReference.h"
+#include "nsTArray.h"
+#include "nsCycleCollectionNoteChild.h"
+
+// nsMaybeWeakPtr is a helper object to hold a strong-or-weak reference
+// to the template class.  It's pretty minimal, but sufficient.
+
+class nsMaybeWeakPtr_base
+{
+protected:
+  // Returns an addref'd pointer to the requested interface
+  void* GetValueAs(const nsIID& iid) const;
+
+  nsCOMPtr<nsISupports> mPtr;
+};
+
+template<class T>
+class nsMaybeWeakPtr : private nsMaybeWeakPtr_base
+{
+public:
+  nsMaybeWeakPtr(nsISupports *ref) { mPtr = ref; }
+  nsMaybeWeakPtr(const nsCOMPtr<nsIWeakReference> &ref) { mPtr = ref; }
+  nsMaybeWeakPtr(const nsCOMPtr<T> &ref) { mPtr = ref; }
+
+  bool operator==(const nsMaybeWeakPtr<T> &other) const {
+    return mPtr == other.mPtr;
+  }
+
+  operator const nsCOMPtr<T>() const { return GetValue(); }
+
+protected:
+  const nsCOMPtr<T> GetValue() const {
+    return nsCOMPtr<T>(dont_AddRef(static_cast<T*>
+                                              (GetValueAs(NS_GET_TEMPLATE_IID(T)))));
+  }
+};
+
+// nsMaybeWeakPtrArray is an array of MaybeWeakPtr objects, that knows how to
+// grab a weak reference to a given object if requested.  It only allows a
+// given object to appear in the array once.
+
+typedef nsTArray< nsMaybeWeakPtr<nsISupports> > isupports_array_type;
+nsresult NS_AppendWeakElementBase(isupports_array_type *aArray,
+                                  nsISupports *aElement, bool aWeak);
+nsresult NS_RemoveWeakElementBase(isupports_array_type *aArray,
+                                  nsISupports *aElement);
+
+template<class T>
+class nsMaybeWeakPtrArray : public nsTArray< nsMaybeWeakPtr<T> >
+{
+public:
+  nsresult AppendWeakElement(T *aElement, bool aOwnsWeak)
+  {
+    return NS_AppendWeakElementBase(
+      reinterpret_cast<isupports_array_type*>(this), aElement, aOwnsWeak);
+  }
+
+  nsresult RemoveWeakElement(T *aElement)
+  {
+    return NS_RemoveWeakElementBase(
+      reinterpret_cast<isupports_array_type*>(this), aElement);
+  }
+};
+
+template <typename T>
+inline void
+ImplCycleCollectionUnlink(nsMaybeWeakPtrArray<T>& aField)
+{
+  aField.Clear();
+}
+
+template <typename E>
+inline void
+ImplCycleCollectionTraverse(nsCycleCollectionTraversalCallback& aCallback,
+                            nsMaybeWeakPtrArray<E>& aField,
+                            const char* aName,
+                            uint32_t aFlags = 0)
+{
+  aFlags |= CycleCollectionEdgeNameArrayFlag;
+  size_t length = aField.Length();
+  for (size_t i = 0; i < length; ++i) {
+    CycleCollectionNoteChild(aCallback, aField[i].get(), aName, aFlags);
+  }
+}
+
+// Call a method on each element in the array, but only if the element is
+// non-null.
+
+#define ENUMERATE_WEAKARRAY(array, type, method)                           \
+  for (uint32_t array_idx = 0; array_idx < array.Length(); ++array_idx) {  \
+    const nsCOMPtr<type> &e = array.ElementAt(array_idx);                  \
+    if (e)                                                                 \
+      e->method;                                                           \
+  }
+
+#endif