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diff --git a/xpcom/string/nsTPromiseFlatString.h b/xpcom/string/nsTPromiseFlatString.h
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+++ b/xpcom/string/nsTPromiseFlatString.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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);
+ *     // Temporaries usually die after the full expression containing the
+ *     // expression that created the temporary is evaluated.  But when a
+ *     // temporary is assigned to a local reference, the temporary's lifetime
+ *     // is extended to the reference's lifetime (C++11 [class.temporary]p5).
+ *     //
+ *     // This reference holds the anonymous temporary alive.  But remember: it
+ *     // must _still_ have a lifetime shorter than that of |aCSubstring|, and
+ *     // |aCSubstring| must not be changed while the PromiseFlatString lives.
+ *
+ *  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&) = delete;
+
+  // NOT TO BE IMPLEMENTED
+  nsTPromiseFlatString_CharT() = delete;
+
+  // NOT TO BE IMPLEMENTED
+  nsTPromiseFlatString_CharT(const string_type& aStr) = delete;
+
+public:
+
+  explicit
+  nsTPromiseFlatString_CharT(const substring_type& aStr)
+    : string_type()
+  {
+    Init(aStr);
+  }
+
+  explicit
+  nsTPromiseFlatString_CharT(const substring_tuple_type& aTuple)
+    : string_type()
+  {
+    // nothing else to do here except assign the value of the tuple
+    // into ourselves.
+    Assign(aTuple);
+  }
+};
+
+// 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& aString)
+{
+  return nsTPromiseFlatString_CharT(aString);
+}