diff -r 000000000000 -r 6474c204b198 xpcom/string/public/nsTPromiseFlatString.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/string/public/nsTPromiseFlatString.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,106 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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/. */
+
+
+  /**
+   * NOTE:
+   *
+   * Try to avoid flat strings.  |PromiseFlat[C]String| will help you as a last
+   * resort, and this may be necessary when dealing with legacy or OS calls,
+   * but in general, requiring a null-terminated array of characters kills many
+   * of the performance wins the string classes offer.  Write your own code to
+   * use |nsA[C]String&|s for parameters.  Write your string proccessing
+   * algorithms to exploit iterators.  If you do this, you will benefit from
+   * being able to chain operations without copying or allocating and your code
+   * will be significantly more efficient.  Remember, a function that takes an
+   * |const nsA[C]String&| can always be passed a raw character pointer by
+   * wrapping it (for free) in a |nsDependent[C]String|.  But a function that
+   * takes a character pointer always has the potential to force allocation and
+   * copying.
+   *
+   *
+   * How to use it:
+   *
+   * A |nsPromiseFlat[C]String| doesn't necessarily own the characters it
+   * promises.  You must never use it to promise characters out of a string
+   * with a shorter lifespan.  The typical use will be something like this:
+   *
+   *   SomeOSFunction( PromiseFlatCString(aCSubstring).get() ); // GOOD
+   *
+   * Here's a BAD use:
+   *
+   *  const char* buffer = PromiseFlatCString(aCSubstring).get();
+   *  SomeOSFunction(buffer); // BAD!! |buffer| is a dangling pointer
+   *
+   * The only way to make one is with the function |PromiseFlat[C]String|,
+   * which produce a |const| instance.  ``What if I need to keep a promise
+   * around for a little while?'' you might ask.  In that case, you can keep a
+   * reference, like so
+   *
+   *   const nsCString& flat = PromiseFlatString(aCSubstring);
+   *     // this reference holds the anonymous temporary alive, but remember,
+   *     // it must _still_ have a lifetime shorter than that of |aCSubstring|
+   *
+   *  SomeOSFunction(flat.get());
+   *  SomeOtherOSFunction(flat.get());
+   *
+   *
+   * How does it work?
+   *
+   * A |nsPromiseFlat[C]String| is just a wrapper for another string.  If you
+   * apply it to a string that happens to be flat, your promise is just a
+   * dependent reference to the string's data.  If you apply it to a non-flat
+   * string, then a temporary flat string is created for you, by allocating and
+   * copying.  In the event that you end up assigning the result into a sharing
+   * string (e.g., |nsTString|), the right thing happens.
+   */
+
+class nsTPromiseFlatString_CharT : public nsTString_CharT
+  {
+    public:
+
+      typedef nsTPromiseFlatString_CharT    self_type;
+
+    private:
+
+      void Init( const substring_type& );
+
+        // NOT TO BE IMPLEMENTED
+      void operator=( const self_type& ) MOZ_DELETE;
+
+        // NOT TO BE IMPLEMENTED
+      nsTPromiseFlatString_CharT() MOZ_DELETE;
+
+        // NOT TO BE IMPLEMENTED
+      nsTPromiseFlatString_CharT( const string_type& str ) MOZ_DELETE;
+
+    public:
+
+      explicit
+      nsTPromiseFlatString_CharT( const substring_type& str )
+        : string_type()
+        {
+          Init(str);
+        }
+
+      explicit
+      nsTPromiseFlatString_CharT( const substring_tuple_type& tuple )
+        : string_type()
+        {
+          // nothing else to do here except assign the value of the tuple
+          // into ourselves.
+          Assign(tuple);
+        }
+  };
+
+// We template this so that the constructor is chosen based on the type of the
+// parameter. This allows us to reject attempts to promise a flat flat string.
+template<class T>
+const nsTPromiseFlatString_CharT
+TPromiseFlatString_CharT( const T& string )
+  {
+    return nsTPromiseFlatString_CharT(string);
+  }