1.1 --- /dev/null Thu Jan 01 00:00:00 1970 +0000
1.2 +++ b/xpcom/string/public/nsTPromiseFlatString.h Wed Dec 31 06:09:35 2014 +0100
1.3 @@ -0,0 +1,106 @@
1.4 +/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
1.5 +/* vim:set ts=2 sw=2 sts=2 et cindent: */
1.6 +/* This Source Code Form is subject to the terms of the Mozilla Public
1.7 + * License, v. 2.0. If a copy of the MPL was not distributed with this
1.8 + * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
1.9 +
1.10 +
1.11 + /**
1.12 + * NOTE:
1.13 + *
1.14 + * Try to avoid flat strings. |PromiseFlat[C]String| will help you as a last
1.15 + * resort, and this may be necessary when dealing with legacy or OS calls,
1.16 + * but in general, requiring a null-terminated array of characters kills many
1.17 + * of the performance wins the string classes offer. Write your own code to
1.18 + * use |nsA[C]String&|s for parameters. Write your string proccessing
1.19 + * algorithms to exploit iterators. If you do this, you will benefit from
1.20 + * being able to chain operations without copying or allocating and your code
1.21 + * will be significantly more efficient. Remember, a function that takes an
1.22 + * |const nsA[C]String&| can always be passed a raw character pointer by
1.23 + * wrapping it (for free) in a |nsDependent[C]String|. But a function that
1.24 + * takes a character pointer always has the potential to force allocation and
1.25 + * copying.
1.26 + *
1.27 + *
1.28 + * How to use it:
1.29 + *
1.30 + * A |nsPromiseFlat[C]String| doesn't necessarily own the characters it
1.31 + * promises. You must never use it to promise characters out of a string
1.32 + * with a shorter lifespan. The typical use will be something like this:
1.33 + *
1.34 + * SomeOSFunction( PromiseFlatCString(aCSubstring).get() ); // GOOD
1.35 + *
1.36 + * Here's a BAD use:
1.37 + *
1.38 + * const char* buffer = PromiseFlatCString(aCSubstring).get();
1.39 + * SomeOSFunction(buffer); // BAD!! |buffer| is a dangling pointer
1.40 + *
1.41 + * The only way to make one is with the function |PromiseFlat[C]String|,
1.42 + * which produce a |const| instance. ``What if I need to keep a promise
1.43 + * around for a little while?'' you might ask. In that case, you can keep a
1.44 + * reference, like so
1.45 + *
1.46 + * const nsCString& flat = PromiseFlatString(aCSubstring);
1.47 + * // this reference holds the anonymous temporary alive, but remember,
1.48 + * // it must _still_ have a lifetime shorter than that of |aCSubstring|
1.49 + *
1.50 + * SomeOSFunction(flat.get());
1.51 + * SomeOtherOSFunction(flat.get());
1.52 + *
1.53 + *
1.54 + * How does it work?
1.55 + *
1.56 + * A |nsPromiseFlat[C]String| is just a wrapper for another string. If you
1.57 + * apply it to a string that happens to be flat, your promise is just a
1.58 + * dependent reference to the string's data. If you apply it to a non-flat
1.59 + * string, then a temporary flat string is created for you, by allocating and
1.60 + * copying. In the event that you end up assigning the result into a sharing
1.61 + * string (e.g., |nsTString|), the right thing happens.
1.62 + */
1.63 +
1.64 +class nsTPromiseFlatString_CharT : public nsTString_CharT
1.65 + {
1.66 + public:
1.67 +
1.68 + typedef nsTPromiseFlatString_CharT self_type;
1.69 +
1.70 + private:
1.71 +
1.72 + void Init( const substring_type& );
1.73 +
1.74 + // NOT TO BE IMPLEMENTED
1.75 + void operator=( const self_type& ) MOZ_DELETE;
1.76 +
1.77 + // NOT TO BE IMPLEMENTED
1.78 + nsTPromiseFlatString_CharT() MOZ_DELETE;
1.79 +
1.80 + // NOT TO BE IMPLEMENTED
1.81 + nsTPromiseFlatString_CharT( const string_type& str ) MOZ_DELETE;
1.82 +
1.83 + public:
1.84 +
1.85 + explicit
1.86 + nsTPromiseFlatString_CharT( const substring_type& str )
1.87 + : string_type()
1.88 + {
1.89 + Init(str);
1.90 + }
1.91 +
1.92 + explicit
1.93 + nsTPromiseFlatString_CharT( const substring_tuple_type& tuple )
1.94 + : string_type()
1.95 + {
1.96 + // nothing else to do here except assign the value of the tuple
1.97 + // into ourselves.
1.98 + Assign(tuple);
1.99 + }
1.100 + };
1.101 +
1.102 +// We template this so that the constructor is chosen based on the type of the
1.103 +// parameter. This allows us to reject attempts to promise a flat flat string.
1.104 +template<class T>
1.105 +const nsTPromiseFlatString_CharT
1.106 +TPromiseFlatString_CharT( const T& string )
1.107 + {
1.108 + return nsTPromiseFlatString_CharT(string);
1.109 + }
