diff -r 000000000000 -r 6474c204b198 xpcom/glue/nsArrayUtils.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/glue/nsArrayUtils.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,40 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* 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 nsArrayUtils_h__
+#define nsArrayUtils_h__
+
+#include "nsCOMPtr.h"
+#include "nsIArray.h"
+
+// helper class for do_QueryElementAt
+class NS_COM_GLUE nsQueryArrayElementAt : public nsCOMPtr_helper
+  {
+    public:
+      nsQueryArrayElementAt(nsIArray* aArray, uint32_t aIndex,
+                            nsresult* aErrorPtr)
+          : mArray(aArray),
+            mIndex(aIndex),
+            mErrorPtr(aErrorPtr)
+        {
+          // nothing else to do here
+        }
+
+      virtual nsresult NS_FASTCALL operator()(const nsIID& aIID, void**) const;
+
+    private:
+      nsIArray*  mArray;
+      uint32_t   mIndex;
+      nsresult*  mErrorPtr;
+  };
+
+inline
+const nsQueryArrayElementAt
+do_QueryElementAt(nsIArray* aArray, uint32_t aIndex, nsresult* aErrorPtr = 0)
+  {
+    return nsQueryArrayElementAt(aArray, aIndex, aErrorPtr);
+  }
+
+#endif // nsArrayUtils_h__