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diff --git a/xpcom/string/nsTPromiseFlatString.h 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);
+}