diff -r 000000000000 -r 6474c204b198 xpcom/string/public/nsPrintfCString.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/string/public/nsPrintfCString.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,41 @@
+/* -*- 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/. */
+
+#ifndef nsPrintfCString_h___
+#define nsPrintfCString_h___
+ 
+#include "nsString.h"
+
+/**
+ * nsPrintfCString lets you create a nsCString using a printf-style format
+ * string.  For example:
+ *
+ *   NS_WARNING(nsPrintfCString("Unexpected value: %f", 13.917).get());
+ *
+ * nsPrintfCString has a small built-in auto-buffer.  For larger strings, it
+ * will allocate on the heap.
+ *
+ * See also nsCString::AppendPrintf().
+ */
+class nsPrintfCString : public nsFixedCString
+{
+  typedef nsCString string_type;
+
+public:
+  explicit nsPrintfCString( const char_type* format, ... )
+    : nsFixedCString(mLocalBuffer, kLocalBufferSize, 0)
+  {
+    va_list ap;
+    va_start(ap, format);
+    AppendPrintf(format, ap);
+    va_end(ap);
+  }
+
+private:
+  static const uint32_t kLocalBufferSize = 16;
+  char_type mLocalBuffer[kLocalBufferSize];
+};
+
+#endif // !defined(nsPrintfCString_h___)