diff options
Diffstat (limited to 'libs/program_options/doc/html/program_options/design.html')
| -rw-r--r-- | libs/program_options/doc/html/program_options/design.html | 202 |
1 files changed, 0 insertions, 202 deletions
diff --git a/libs/program_options/doc/html/program_options/design.html b/libs/program_options/doc/html/program_options/design.html deleted file mode 100644 index 9591f0c721..0000000000 --- a/libs/program_options/doc/html/program_options/design.html +++ /dev/null @@ -1,202 +0,0 @@ -<html> -<head> -<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> -<title>Design Discussion</title> -<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css"> -<meta name="generator" content="DocBook XSL Stylesheets V1.76.1"> -<link rel="home" href="../index.html" title="Chapter 1. Boost.Program_options"> -<link rel="up" href="../index.html" title="Chapter 1. Boost.Program_options"> -<link rel="prev" href="howto.html" title="How To"> -<link rel="next" href="../index/s06.html" title="Acknowledgements"> -</head> -<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> -<table cellpadding="2" width="100%"><tr> -<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td> -<td align="center"><a href="../../../../../index.html">Home</a></td> -<td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td> -<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> -<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> -<td align="center"><a href="../../../../../more/index.htm">More</a></td> -</tr></table> -<hr> -<div class="spirit-nav"> -<a accesskey="p" href="howto.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../index/s06.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> -</div> -<div class="section"> -<div class="titlepage"><div><div><h2 class="title" style="clear: both"> -<a name="program_options.design"></a>Design Discussion</h2></div></div></div> -<div class="toc"><dl><dt><span class="section"><a href="design.html#program_options.design.unicode">Unicode Support</a></span></dt></dl></div> -<p>This section focuses on some of the design questions. - </p> -<div class="section"> -<div class="titlepage"><div><div><h3 class="title"> -<a name="program_options.design.unicode"></a>Unicode Support</h3></div></div></div> -<p>Unicode support was one of the features specifically requested - during the formal review. Throughout this document "Unicode support" is - a synonym for "wchar_t" support, assuming that "wchar_t" always uses - Unicode encoding. Also, when talking about "ascii" (in lowercase) we'll - not mean strict 7-bit ASCII encoding, but rather "char" strings in local - 8-bit encoding. - </p> -<p> - Generally, "Unicode support" can mean - many things, but for the program_options library it means that: - - </p> -<div class="itemizedlist"><ul class="itemizedlist" type="disc"> -<li class="listitem"><p>Each parser should accept either <code class="computeroutput">char*</code> - or <code class="computeroutput">wchar_t*</code>, correctly split the input into option - names and option values and return the data. - </p></li> -<li class="listitem"><p>For each option, it should be possible to specify whether the conversion - from string to value uses ascii or Unicode. - </p></li> -<li class="listitem"> -<p>The library guarantees that: - </p> -<div class="itemizedlist"><ul class="itemizedlist" type="circle"> -<li class="listitem"><p>ascii input is passed to an ascii value without change - </p></li> -<li class="listitem"><p>Unicode input is passed to a Unicode value without change</p></li> -<li class="listitem"><p>ascii input passed to a Unicode value, and Unicode input - passed to an ascii value will be converted using a codecvt - facet (which may be specified by the user). - </p></li> -</ul></div> -<p> - </p> -</li> -</ul></div> -<p> - </p> -<p>The important point is that it's possible to have some "ascii - options" together with "Unicode options". There are two reasons for - this. First, for a given type you might not have the code to extract the - value from Unicode string and it's not good to require that such code be written. - Second, imagine a reusable library which has some options and exposes - options description in its interface. If <span class="emphasis"><em>all</em></span> - options are either ascii or Unicode, and the library does not use any - Unicode strings, then the author is likely to use ascii options, making - the library unusable inside Unicode - applications. Essentially, it would be necessary to provide two versions - of the library -- ascii and Unicode. - </p> -<p>Another important point is that ascii strings are passed though - without modification. In other words, it's not possible to just convert - ascii to Unicode and process the Unicode further. The problem is that the - default conversion mechanism -- the <code class="computeroutput">codecvt</code> facet -- might - not work with 8-bit input without additional setup. - </p> -<p>The Unicode support outlined above is not complete. For example, we - don't support Unicode option names. Unicode support is hard and - requires a Boost-wide solution. Even comparing two arbitrary Unicode - strings is non-trivial. Finally, using Unicode in option names is - related to internationalization, which has it's own - complexities. E.g. if option names depend on current locale, then all - program parts and other parts which use the name must be - internationalized too. - </p> -<p>The primary question in implementing the Unicode support is whether - to use templates and <code class="computeroutput">std::basic_string</code> or to use some - internal encoding and convert between internal and external encodings on - the interface boundaries. - </p> -<p>The choice, mostly, is between code size and execution - speed. A templated solution would either link library code into every - application that uses the library (thereby making shared library - impossible), or provide explicit instantiations in the shared library - (increasing its size). The solution based on internal encoding would - necessarily make conversions in a number of places and will be somewhat slower. - Since speed is generally not an issue for this library, the second - solution looks more attractive, but we'll take a closer look at - individual components. - </p> -<p>For the parsers component, we have three choices: - </p> -<div class="itemizedlist"><ul class="itemizedlist" type="disc"> -<li class="listitem"><p>Use a fully templated implementation: given a string of a - certain type, a parser will return a <code class="computeroutput"><a class="link" href="reference.html#boost.program_options.parsed_options">parsed_options</a></code> instance - with strings of the same type (i.e. the <code class="computeroutput"><a class="link" href="reference.html#boost.program_options.parsed_options">parsed_options</a></code> class - will be templated).</p></li> -<li class="listitem"><p>Use internal encoding: same as above, but strings will be converted to and - from the internal encoding.</p></li> -<li class="listitem"><p>Use and partly expose the internal encoding: same as above, - but the strings in the <code class="computeroutput"><a class="link" href="reference.html#boost.program_options.parsed_options">parsed_options</a></code> instance will be in the - internal encoding. This might avoid a conversion if - <code class="computeroutput"><a class="link" href="reference.html#boost.program_options.parsed_options">parsed_options</a></code> instance is passed directly to other components, - but can be also dangerous or confusing for a user. - </p></li> -</ul></div> -<p> - </p> -<p>The second solution appears to be the best -- it does not increase - the code size much and is cleaner than the third. To avoid extra - conversions, the Unicode version of <code class="computeroutput"><a class="link" href="reference.html#boost.program_options.parsed_options">parsed_options</a></code> can also store - strings in internal encoding. - </p> -<p>For the options descriptions component, we don't have much - choice. Since it's not desirable to have either all options use ascii or all - of them use Unicode, but rather have some ascii and some Unicode options, the - interface of the <code class="computeroutput"><a class="link" href="../boost/program_options/value_semantic.html" title="Class value_semantic">value_semantic</a></code> must work with both. The only way is - to pass an additional flag telling if strings use ascii or internal encoding. - The instance of <code class="computeroutput"><a class="link" href="../boost/program_options/value_semantic.html" title="Class value_semantic">value_semantic</a></code> can then convert into some - other encoding if needed. - </p> -<p>For the storage component, the only affected function is <code class="computeroutput"><a class="link" href="../boost/program_options/store_idp6705280.html" title="Function store">store</a></code>. - For Unicode input, the <code class="computeroutput"><a class="link" href="../boost/program_options/store_idp6705280.html" title="Function store">store</a></code> function should convert the value to the - internal encoding. It should also inform the <code class="computeroutput"><a class="link" href="../boost/program_options/value_semantic.html" title="Class value_semantic">value_semantic</a></code> class - about the used encoding. - </p> -<p>Finally, what internal encoding should we use? The - alternatives are: - <code class="computeroutput">std::wstring</code> (using UCS-4 encoding) and - <code class="computeroutput">std::string</code> (using UTF-8 encoding). The difference between - alternatives is: - </p> -<div class="itemizedlist"><ul class="itemizedlist" type="disc"> -<li class="listitem"><p>Speed: UTF-8 is a bit slower</p></li> -<li class="listitem"><p>Space: UTF-8 takes less space when input is ascii</p></li> -<li class="listitem"><p>Code size: UTF-8 requires additional conversion code. However, - it allows one to use existing parsers without converting them to - <code class="computeroutput">std::wstring</code> and such conversion is likely to create a - number of new instantiations. - </p></li> -</ul></div> -<p> - There's no clear leader, but the last point seems important, so UTF-8 - will be used. - </p> -<p>Choosing the UTF-8 encoding allows the use of existing parsers, - because 7-bit ascii characters retain their values in UTF-8, - so searching for 7-bit strings is simple. However, there are - two subtle issues: - </p> -<div class="itemizedlist"><ul class="itemizedlist" type="disc"> -<li class="listitem"><p>We need to assume the character literals use ascii encoding - and that inputs use Unicode encoding.</p></li> -<li class="listitem"><p>A Unicode character (say '=') can be followed by 'composing - character' and the combination is not the same as just '=', so a - simple search for '=' might find the wrong character. - </p></li> -</ul></div> -<p> - Neither of these issues appear to be critical in practice, since ascii is - almost universal encoding and since composing characters following '=' (and - other characters with special meaning to the library) are not likely to appear. - </p> -</div> -</div> -<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> -<td align="left"></td> -<td align="right"><div class="copyright-footer">Copyright © 2002-2004 Vladimir Prus<p>Distributed under the Boost Software License, Version 1.0. - (See accompanying file <code class="filename">LICENSE_1_0.txt</code> or copy at - <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) - </p> -</div></td> -</tr></table> -<hr> -<div class="spirit-nav"> -<a accesskey="p" href="howto.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../index/s06.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> -</div> -</body> -</html> |