The Tor Browser: comparison xpcom/string/public/nsTPromiseFlatString.h

--1:000000000000
+:7a66354dc0bc
+/* -*- 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);
+}

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comparison: xpcom/string/public/nsTPromiseFlatString.h

xpcom/string/public/nsTPromiseFlatString.h