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113 lines
4.0 KiB
C++
113 lines
4.0 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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/**
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* NOTE:
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*
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* Try to avoid flat strings. |PromiseFlat[C]String| will help you as a last
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* resort, and this may be necessary when dealing with legacy or OS calls,
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* but in general, requiring a null-terminated array of characters kills many
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* of the performance wins the string classes offer. Write your own code to
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* use |nsA[C]String&|s for parameters. Write your string proccessing
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* algorithms to exploit iterators. If you do this, you will benefit from
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* being able to chain operations without copying or allocating and your code
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* will be significantly more efficient. Remember, a function that takes an
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* |const nsA[C]String&| can always be passed a raw character pointer by
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* wrapping it (for free) in a |nsDependent[C]String|. But a function that
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* takes a character pointer always has the potential to force allocation and
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* copying.
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*
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*
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* How to use it:
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*
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* A |nsPromiseFlat[C]String| doesn't necessarily own the characters it
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* promises. You must never use it to promise characters out of a string
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* with a shorter lifespan. The typical use will be something like this:
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*
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* SomeOSFunction( PromiseFlatCString(aCSubstring).get() ); // GOOD
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*
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* Here's a BAD use:
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*
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* const char* buffer = PromiseFlatCString(aCSubstring).get();
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* SomeOSFunction(buffer); // BAD!! |buffer| is a dangling pointer
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*
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* The only way to make one is with the function |PromiseFlat[C]String|,
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* which produce a |const| instance. ``What if I need to keep a promise
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* around for a little while?'' you might ask. In that case, you can keep a
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* reference, like so:
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*
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* const nsCString& flat = PromiseFlatString(aCSubstring);
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* // Temporaries usually die after the full expression containing the
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* // expression that created the temporary is evaluated. But when a
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* // temporary is assigned to a local reference, the temporary's lifetime
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* // is extended to the reference's lifetime (C++11 [class.temporary]p5).
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* //
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* // This reference holds the anonymous temporary alive. But remember: it
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* // must _still_ have a lifetime shorter than that of |aCSubstring|, and
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* // |aCSubstring| must not be changed while the PromiseFlatString lives.
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*
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* SomeOSFunction(flat.get());
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* SomeOtherOSFunction(flat.get());
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*
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*
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* How does it work?
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*
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* A |nsPromiseFlat[C]String| is just a wrapper for another string. If you
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* apply it to a string that happens to be flat, your promise is just a
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* dependent reference to the string's data. If you apply it to a non-flat
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* string, then a temporary flat string is created for you, by allocating and
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* copying. In the event that you end up assigning the result into a sharing
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* string (e.g., |nsTString|), the right thing happens.
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*/
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class nsTPromiseFlatString_CharT : public nsTString_CharT
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{
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public:
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typedef nsTPromiseFlatString_CharT self_type;
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private:
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void Init(const substring_type&);
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// NOT TO BE IMPLEMENTED
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void operator=(const self_type&) MOZ_DELETE;
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// NOT TO BE IMPLEMENTED
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nsTPromiseFlatString_CharT() MOZ_DELETE;
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// NOT TO BE IMPLEMENTED
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nsTPromiseFlatString_CharT(const string_type& aStr) MOZ_DELETE;
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public:
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explicit
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nsTPromiseFlatString_CharT(const substring_type& aStr)
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: string_type()
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{
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Init(aStr);
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}
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explicit
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nsTPromiseFlatString_CharT(const substring_tuple_type& aTuple)
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: string_type()
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{
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// nothing else to do here except assign the value of the tuple
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// into ourselves.
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Assign(aTuple);
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}
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};
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// We template this so that the constructor is chosen based on the type of the
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// parameter. This allows us to reject attempts to promise a flat flat string.
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template<class T>
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const nsTPromiseFlatString_CharT
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TPromiseFlatString_CharT(const T& aString)
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{
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return nsTPromiseFlatString_CharT(aString);
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}
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