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xpcom/string/public/nsTPromiseFlatString.h@6474c204b198 (annotated)

xpcom/string/public/nsTPromiseFlatString.h

Wed, 31 Dec 2014 06:09:35 +0100

author

Michael Schloh von Bennewitz <michael@schloh.com>

date

Wed, 31 Dec 2014 06:09:35 +0100

changeset 0

6474c204b198

permissions

-rw-r--r--

Cloned upstream origin tor-browser at tor-browser-31.3.0esr-4.5-1-build1
revision ID fc1c9ff7c1b2defdbc039f12214767608f46423f for hacking purpose.

michael@0	1	/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
michael@0	2	/* vim:set ts=2 sw=2 sts=2 et cindent: */
michael@0	3	/* This Source Code Form is subject to the terms of the Mozilla Public
michael@0	4	* License, v. 2.0. If a copy of the MPL was not distributed with this
michael@0	5	* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
michael@0	6
michael@0	7
michael@0	8	/**
michael@0	9	* NOTE:
michael@0	10	*
michael@0	11	* Try to avoid flat strings. |PromiseFlat[C]String| will help you as a last
michael@0	12	* resort, and this may be necessary when dealing with legacy or OS calls,
michael@0	13	* but in general, requiring a null-terminated array of characters kills many
michael@0	14	* of the performance wins the string classes offer. Write your own code to
michael@0	15	* use |nsA[C]String&|s for parameters. Write your string proccessing
michael@0	16	* algorithms to exploit iterators. If you do this, you will benefit from
michael@0	17	* being able to chain operations without copying or allocating and your code
michael@0	18	* will be significantly more efficient. Remember, a function that takes an
michael@0	19	* |const nsA[C]String&| can always be passed a raw character pointer by
michael@0	20	* wrapping it (for free) in a |nsDependent[C]String|. But a function that
michael@0	21	* takes a character pointer always has the potential to force allocation and
michael@0	22	* copying.
michael@0	23	*
michael@0	24	*
michael@0	25	* How to use it:
michael@0	26	*
michael@0	27	* A |nsPromiseFlat[C]String| doesn't necessarily own the characters it
michael@0	28	* promises. You must never use it to promise characters out of a string
michael@0	29	* with a shorter lifespan. The typical use will be something like this:
michael@0	30	*
michael@0	31	* SomeOSFunction( PromiseFlatCString(aCSubstring).get() ); // GOOD
michael@0	32	*
michael@0	33	* Here's a BAD use:
michael@0	34	*
michael@0	35	* const char* buffer = PromiseFlatCString(aCSubstring).get();
michael@0	36	* SomeOSFunction(buffer); // BAD!! |buffer| is a dangling pointer
michael@0	37	*
michael@0	38	* The only way to make one is with the function |PromiseFlat[C]String|,
michael@0	39	* which produce a |const| instance. ``What if I need to keep a promise
michael@0	40	* around for a little while?'' you might ask. In that case, you can keep a
michael@0	41	* reference, like so
michael@0	42	*
michael@0	43	* const nsCString& flat = PromiseFlatString(aCSubstring);
michael@0	44	* // this reference holds the anonymous temporary alive, but remember,
michael@0	45	* // it must _still_ have a lifetime shorter than that of |aCSubstring|
michael@0	46	*
michael@0	47	* SomeOSFunction(flat.get());
michael@0	48	* SomeOtherOSFunction(flat.get());
michael@0	49	*
michael@0	50	*
michael@0	51	* How does it work?
michael@0	52	*
michael@0	53	* A |nsPromiseFlat[C]String| is just a wrapper for another string. If you
michael@0	54	* apply it to a string that happens to be flat, your promise is just a
michael@0	55	* dependent reference to the string's data. If you apply it to a non-flat
michael@0	56	* string, then a temporary flat string is created for you, by allocating and
michael@0	57	* copying. In the event that you end up assigning the result into a sharing
michael@0	58	* string (e.g., |nsTString|), the right thing happens.
michael@0	59	*/
michael@0	60
michael@0	61	class nsTPromiseFlatString_CharT : public nsTString_CharT
michael@0	62	{
michael@0	63	public:
michael@0	64
michael@0	65	typedef nsTPromiseFlatString_CharT self_type;
michael@0	66
michael@0	67	private:
michael@0	68
michael@0	69	void Init( const substring_type& );
michael@0	70
michael@0	71	// NOT TO BE IMPLEMENTED
michael@0	72	void operator=( const self_type& ) MOZ_DELETE;
michael@0	73
michael@0	74	// NOT TO BE IMPLEMENTED
michael@0	75	nsTPromiseFlatString_CharT() MOZ_DELETE;
michael@0	76
michael@0	77	// NOT TO BE IMPLEMENTED
michael@0	78	nsTPromiseFlatString_CharT( const string_type& str ) MOZ_DELETE;
michael@0	79
michael@0	80	public:
michael@0	81
michael@0	82	explicit
michael@0	83	nsTPromiseFlatString_CharT( const substring_type& str )
michael@0	84	: string_type()
michael@0	85	{
michael@0	86	Init(str);
michael@0	87	}
michael@0	88
michael@0	89	explicit
michael@0	90	nsTPromiseFlatString_CharT( const substring_tuple_type& tuple )
michael@0	91	: string_type()
michael@0	92	{
michael@0	93	// nothing else to do here except assign the value of the tuple
michael@0	94	// into ourselves.
michael@0	95	Assign(tuple);
michael@0	96	}
michael@0	97	};
michael@0	98
michael@0	99	// We template this so that the constructor is chosen based on the type of the
michael@0	100	// parameter. This allows us to reject attempts to promise a flat flat string.
michael@0	101	template<class T>
michael@0	102	const nsTPromiseFlatString_CharT
michael@0	103	TPromiseFlatString_CharT( const T& string )
michael@0	104	{
michael@0	105	return nsTPromiseFlatString_CharT(string);
michael@0	106	}