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<a name="sec:utf8-codecvt-facet-class"></a>
<h1><code>utf8_codecvt_facet</code></h1>
<pre>
template<
typename InternType = wchar_t,
typename ExternType = char
> utf8_codecvt_facet
</pre>
<h2>Rationale</h2>
UTF-8 is a method of encoding Unicode text in environments where
where data is stored as 8-bit characters and some ascii characters
are considered special (i.e. Unix filesystem filenames) and tend
to appear more commonly than other characters. While
UTF-8 is convenient and efficient for storing data on filesystems,
it was not meant to be manipulated in memory by
applications. While some applications (such as Unix's 'cat') can
simply ignore the encoding of data, others should convert
from UTF-8 to UCS-4 (the more canonical representation of Unicode)
on reading from file, and reversing the process on writing out to
file.
<p>The C++ Standard IOStreams provides the <tt>std::codecvt</tt>
facet to handle specifically these cases. On reading from or
writing to a file, the <tt>std::basic_filebuf</tt> can call out to
the codecvt facet to convert data representations from external
format (ie. UTF-8) to internal format (ie. UCS-4) and
vice-versa. <tt>utf8_codecvt_facet</tt> is a specialization of
<tt>std::codecvt</tt> specifically designed to handle the case
of translating between UTF-8 and UCS-4.
<h2>Template Parameters</h2>
<table border summary="template parameters">
<tr>
<th>Parameter</th><th>Description</th><th>Default</th>
</tr>
<tr>
<td><tt>InternType</tt></td>
<td>The internal type used to represent UCS-4 characters.</td>
<td><tt>wchar_t</tt></td>
</tr>
<tr>
<td><tt>ExternType</tt></td>
<td>The external type used to represent UTF-8 octets.</td>
<td><tt>char_t</tt></td>
</tr>
</table>
<h2>Requirements</h2>
<tt>utf8_codecvt_facet</tt> defaults to using <tt>char</tt> as
it's external data type and <tt>wchar_t</tt> as it's internal
datatype, but on some architectures <tt>wchar_t</tt> is
not large enough to hold UCS-4 characters. In order to use
another internal type.You must also specialize <tt>std::codecvt</tt>
to handle your internal and external types.
(<tt>std::codecvt<char,wchar_t,std::mbstate_t></tt> is required to be
supplied by any standard-conforming compiler).
<h2>Example Use</h2>
The following is a simple example of using this facet:
<pre>
//...
// My encoding type
typedef wchar_t ucs4_t;
std::locale old_locale;
std::locale utf8_locale(old_locale,new utf8_codecvt_facet<ucs4_t>);
// Set a New global locale
std::locale::global(utf8_locale);
// Send the UCS-4 data out, converting to UTF-8
{
std::wofstream ofs("data.ucd");
ofs.imbue(utf8_locale);
std::copy(ucs4_data.begin(),ucs4_data.end(),
std::ostream_iterator<ucs4_t,ucs4_t>(ofs));
}
// Read the UTF-8 data back in, converting to UCS-4 on the way in
std::vector<ucs4_t> from_file;
{
std::wifstream ifs("data.ucd");
ifs.imbue(utf8_locale);
ucs4_t item = 0;
while (ifs >> item) from_file.push_back(item);
}
//...
</pre>
<h2>History</h2>
This code was originally written as an iterator adaptor over
containers for use with UTF-8 encoded strings in memory.
Dietmar Kuehl suggested that it would be better provided as a
codecvt facet.
<h2>Resources</h2>
<ul>
<li> <a href="http://www.unicode.org">Unicode Homepage</a>
<li> <a href="http://home.CameloT.de/langer/iostreams.htm">Standard
C++ IOStreams and Locales</a>
<li> <a href="http://www.research.att.com/~bs/3rd.html">The C++
Programming Language Special Edition, Appendix D.</a>
</ul>
<br>
<hr>
<table summary="Copyright information">
<tr valign="top">
<td nowrap>Copyright © 2001</td>
<td><a href="http://www.osl.iu.edu/~garcia">Ronald Garcia</a>,
Indiana University
(<a href="mailto:garcia@cs.indiana.edu">garcia@osl.iu.edu</a>)<br>
<a href="http://www.osl.iu.edu/~lums">Andrew Lumsdaine</a>,
Indiana University
(<a href="mailto:lums@osl.iu.edu">lums@osl.iu.edu</a>)</td>
</tr>
</table>
<p><i>© Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2002-2004.
Distributed under the Boost Software License, Version 1.0. (See
accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
</i></p>
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