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width="20%" align="right"> <a accesskey="n" href="codecvt.html">Next</a></td></tr></table><hr /></div><div class="chapter" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title"><a id="manual.localization.facet"></a>Chapter 15. Facets aka Categories</h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="facets.html#manual.localization.facet.ctype">ctype</a></span></dt><dd><dl><dt><span class="sect2"><a href="facets.html#facet.ctype.impl">Implementation</a></span></dt><dt><span class="sect2"><a href="facets.html#facet.ctype.future">Future</a></span></dt></dl></dd><dt><span class="sect1"><a href="codecvt.html">codecvt</a></span></dt><dd><dl><dt><span class="sect2"><a href="codecvt.html#facet.codecvt.req">Requirements</a></span></dt><dt><span class="sect2"><a href="codecvt.html#facet.codecvt.design">Design</a></span></dt><dt><span class="sect2"><a href="codecvt.html#facet.codecvt.impl">Implementation</a></span></dt><dt><span class="sect2"><a href="codecvt.html#facet.codecvt.use">Use</a></span></dt><dt><span class="sect2"><a href="codecvt.html#facet.codecvt.future">Future</a></span></dt></dl></dd><dt><span class="sect1"><a href="messages.html">messages</a></span></dt><dd><dl><dt><span class="sect2"><a href="messages.html#facet.messages.req">Requirements</a></span></dt><dt><span class="sect2"><a href="messages.html#facet.messages.design">Design</a></span></dt><dt><span class="sect2"><a href="messages.html#facet.messages.impl">Implementation</a></span></dt><dt><span class="sect2"><a href="messages.html#facet.messages.use">Use</a></span></dt><dt><span class="sect2"><a href="messages.html#facet.messages.future">Future</a></span></dt></dl></dd></dl></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.localization.facet.ctype"></a>ctype</h2></div></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="facet.ctype.impl"></a>Implementation</h3></div></div></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id577250"></a>Specializations</h4></div></div></div><p>
+</th><td
align="right"> <a accesskey="n" href="containers.html">Next</a></td></tr></table><hr/></div><div class="section" title="Facets"><div class="titlepage"><div><div><h2 class="title"><a id="std.localization.facet"/>Facets</h2></div></div></div><div class="section" title="ctype"><div class="titlepage"><div><div><h3 class="title"><a id="std.localization.facet.ctype"/>ctype</h3></div></div></div><div class="section" title="Implementation"><div class="titlepage"><div><div><h4 class="title"><a id="facet.ctype.impl"/>Implementation</h4></div></div></div><div class="section" title="Specializations"><div class="titlepage"><div><div><h5 class="title"><a id="id609115"/>Specializations</h5></div></div></div><p>
For the required specialization codecvt<wchar_t, char, mbstate_t> ,
conversions are made between the internal character set (always UCS4
on GNU/Linux) and whatever the currently selected locale for the
</p><p>
Neither of these two required specializations deals with Unicode
characters.
-</p></div></div><div class="sect
2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="facet.ctype.future"></a>Future</h3></div></div></div><div class="itemizedlist"><ul type="disc"><li><p>
- How to deal with the global locale issue?
- </p></li><li><p>
- How to deal with different types than char, wchar_t? </p></li><li><p>
- Overlap between codecvt/ctype: narrow/widen
- </p></li><li><p>
+</p></div></div><div class="sect
ion" title="Future"><div class="titlepage"><div><div><h4 class="title"><a id="facet.ctype.future"/>Future</h4></div></div></div><div class="itemizedlist"><ul class="itemizedlist"><li class="listitem"><p>
+ How to deal with the global locale issue?
+ </p></li><li class="listitem"><p>
+ How to deal with different types than char, wchar_t? </p></li><li
class="listitem"><p>
+ Overlap between codecvt/ctype: narrow/widen
+ </p></li><li class="listitem"><p>
Mask typedef in codecvt_base, argument types in codecvt. what
- is know about this type?
- </p></li><li><p>
+ is know about this type?
+ </p></li><li class="listitem"><p>
Why mask* argument in codecvt?
- </p></li><li><p>
+ </p></li><li
class="listitem"><p>
Can this be made (more) generic? is there a simple way to
straighten out the configure-time mess that is a by-product of
- this class?
- </p></li><li><p>
+ this class?
+ </p></li><li class="listitem"><p>
Get the ctype<wchar_t>::mask stuff under control. Need to
make some kind of static table, and not do lookup every time
somebody hits the do_is... functions. Too bad we can't just
- redefine mask for ctype<wchar_t>
- </p></li><li><p>
+ redefine mask for ctype<wchar_t>
+ </p></li><li class="listitem"><p>
Rename abstract base class. See if just smash-overriding is a
better approach. Clarify, add sanity to naming.
- </p></li></ul></div></div><div class="bibliography"><div class="titlepage"><div><div><h3 class="title"><a id="facet.ctype.biblio"></a>Bibliography</h3></div></div></div><div class="biblioentry"><a id="id596298"></a><p><span class="title"><i>
+ </p></li></ul></div></div><div class="bibliography" title="Bibliography"><div class="titlepage"><div><div><h4 class="title"><a id="facet.ctype.biblio"/>Bibliography</h4></div></div></div><div class="biblioentry"><a id="id609240"/><p><span class="citetitle"><em class="citetitle">
The GNU C Library
- </i>. </span><span class="author"><span class="firstname">Roland</span> <span class="surname">McGrath</span>. </span><span class="author"><span class="firstname">Ulrich</span> <span class="surname">Drepper</span>. </span><span class="copyright">Copyright © 2007 FSF. </span><span class="pagenums">Chapters 6 Character Set Handling and 7 Locales and Internationalization. </span></p></div><div class="biblioentry"><a id="id657031"></a><p><span class="title"><i>
+ </em>. </span><span class="author"><span class="firstname">Roland</span> <span class="surname">McGrath</span>. </span><span class="author"><span class="firstname">Ulrich</span> <span class="surname">Drepper</span>. </span><span class="copyright">Copyright © 2007 FSF. </span><span class="pagenums">Chapters 6 Character Set Handling and 7 Locales and Internationalization. </span></p></div><div class="biblioentry"><a id="id609280"/><p><span class="citetitle"><em class="citetitle">
Correspondence
- </i>. </span><span class="author"><span class="firstname">Ulrich</span> <span class="surname">Drepper</span>. </span><span class="copyright">Copyright © 2002 . </span></p></div><div class="biblioentry"><a id="id646518"></a><p><span class="title"><i>
+ </em>. </span><span class="author"><span class="firstname">Ulrich</span> <span class="surname">Drepper</span>. </span><span class="copyright">Copyright © 2002 . </span></p></div><div class="biblioentry"><a id="id609306"/><p><span class="citetitle"><em class="citetitle">
ISO/IEC 14882:1998 Programming languages - C++
- </i>. </span><span class="copyright">Copyright © 1998 ISO. </span></p></div><div class="biblioentry"><a id="id646537"></a><p><span class="title"><i>
+ </em>. </span><span class="copyright">Copyright © 1998 ISO. </span></p></div><div class="biblioentry"><a id="id609325"/><p><span class="citetitle"><em class="citetitle">
ISO/IEC 9899:1999 Programming languages - C
- </i>. </span><span class="copyright">Copyright © 1999 ISO. </span></p></div><div class="biblioentry"><a id="id587314"></a><p><span class="title"><i>
- System Interface Definitions, Issue 6 (IEEE Std. 1003.1-200x)
- </i>. </span><span class="copyright">Copyright © 1999
- The Open Group/The Institute of Electrical and Electronics Engineers, Inc.. </span><span class="biblioid">
- <a class="ulink" href="http://www.opennc.org/austin/docreg.html" target="_top">
- </a>
- . </span></p></div><div class="biblioentry"><a id="id578920"></a><p><span class="title"><i>
+ </em>. </span><span class="copyright">Copyright © 1999 ISO. </span></p></div><div class="biblioentry" title="The Open Group Base Specifications, Issue 6 (IEEE Std. 1003.1-2004)"><a id="id609344"/><p><span class="title"><em>
+ <a class="link" href="http://www.unix.org/version3/ieee_std.html">
+ The Open Group Base Specifications, Issue 6 (IEEE Std. 1003.1-2004)
+ </a>
+ </em>. </span><span class="copyright">Copyright © 1999
+ The Open Group/The Institute of Electrical and Electronics Engineers, Inc.. </span></p></div><div class="biblioentry"><a id="id609371"/><p><span class="citetitle"><em class="citetitle">
The C++ Programming Language, Special Edition
- </i>. </span><span class="author"><span class="firstname">Bjarne</span> <span class="surname">Stroustrup</span>. </span><span class="copyright">Copyright © 2000 Addison Wesley, Inc.. </span><span class="pagenums">Appendix D. </span><span class="publisher"><span class="publishername">
+ </em>. </span><span class="author"><span class="firstname">Bjarne</span> <span class="surname">Stroustrup</span>. </span><span class="copyright">Copyright © 2000 Addison Wesley, Inc.. </span><span class="pagenums">Appendix D. </span><span class="publisher"><span class="publishername">
Addison Wesley
- . </span></span></p></div><div class="biblioentry"><a id="id655466"></a><p><span class="title"><i>
+ . </span></span></p></div><div class="biblioentry"><a id="id609409"/><p><span class="citetitle"><em class="citetitle">
Standard C++ IOStreams and Locales
- </i>. </span><span class="subtitle">
+ </em>. </span><span class="subtitle">
Advanced Programmer's Guide and Reference
. </span><span class="author"><span class="firstname">Angelika</span> <span class="surname">Langer</span>. </span><span class="author"><span class="firstname">Klaus</span> <span class="surname">Kreft</span>. </span><span class="copyright">Copyright © 2000 Addison Wesley Longman, Inc.. </span><span class="publisher"><span class="publishername">
Addison Wesley Longman
- . </span></span></p></div></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="locales.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="localization.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="codecvt.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 14. Locales </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> codecvt</td></tr></table></div></body></html>
+ . </span></span></p></div></div></div><div class="section" title="codecvt"><div class="titlepage"><div><div><h3 class="title"><a id="std.localization.facet.codecvt"/>codecvt</h3></div></div></div><p>
+The standard class codecvt attempts to address conversions between
+different character encoding schemes. In particular, the standard
+attempts to detail conversions between the implementation-defined wide
+characters (hereafter referred to as wchar_t) and the standard type
+char that is so beloved in classic <span class="quote">“<span class="quote">C</span>”</span> (which can now be
+referred to as narrow characters.) This document attempts to describe
+how the GNU libstdc++ implementation deals with the conversion between
+wide and narrow characters, and also presents a framework for dealing
+with the huge number of other encodings that iconv can convert,
+including Unicode and UTF8. Design issues and requirements are
+addressed, and examples of correct usage for both the required
+specializations for wide and narrow characters and the
+implementation-provided extended functionality are given.
+</p><div class="section" title="Requirements"><div class="titlepage"><div><div><h4 class="title"><a id="facet.codecvt.req"/>Requirements</h4></div></div></div><p>
+Around page 425 of the C++ Standard, this charming heading comes into view:
+</p><div class="blockquote"><blockquote class="blockquote"><p>
+22.2.1.5 - Template class codecvt
+</p></blockquote></div><p>
+The text around the codecvt definition gives some clues:
+</p><div class="blockquote"><blockquote class="blockquote"><p>
+<span class="emphasis"><em>
+-1- The class codecvt<internT,externT,stateT> is for use when
+converting from one codeset to another, such as from wide characters
+to multibyte characters, between wide character encodings such as
+Unicode and EUC.
+</em></span>
+</p></blockquote></div><p>
+Hmm. So, in some unspecified way, Unicode encodings and
+translations between other character sets should be handled by this
+class.
+</p><div class="blockquote"><blockquote class="blockquote"><p>
+<span class="emphasis"><em>
+-2- The stateT argument selects the pair of codesets being mapped between.
+</em></span>
+</p></blockquote></div><p>
+Ah ha! Another clue...
+</p><div class="blockquote"><blockquote class="blockquote"><p>
+<span class="emphasis"><em>
+-3- The instantiations required in the Table ??
+(lib.locale.category), namely codecvt<wchar_t,char,mbstate_t> and
+codecvt<char,char,mbstate_t>, convert the implementation-defined
+native character set. codecvt<char,char,mbstate_t> implements a
+degenerate conversion; it does not convert at
+all. codecvt<wchar_t,char,mbstate_t> converts between the native
+character sets for tiny and wide characters. Instantiations on
+mbstate_t perform conversion between encodings known to the library
+implementor. Other encodings can be converted by specializing on a
+user-defined stateT type. The stateT object can contain any state that
+is useful to communicate to or from the specialized do_convert member.
+</em></span>
+</p></blockquote></div><p>
+At this point, a couple points become clear:
+</p><p>
+One: The standard clearly implies that attempts to add non-required
+(yet useful and widely used) conversions need to do so through the
+third template parameter, stateT.</p><p>
+Two: The required conversions, by specifying mbstate_t as the third
+template parameter, imply an implementation strategy that is mostly
+(or wholly) based on the underlying C library, and the functions
+mcsrtombs and wcsrtombs in particular.</p></div><div class="section" title="Design"><div class="titlepage"><div><div><h4 class="title"><a id="facet.codecvt.design"/>Design</h4></div></div></div><div class="section" title="wchar_t Size"><div class="titlepage"><div><div><h5 class="title"><a id="codecvt.design.wchar_t_size"/><span class="type">wchar_t</span> Size</h5></div></div></div><p>
+ The simple implementation detail of wchar_t's size seems to
+ repeatedly confound people. Many systems use a two byte,
+ unsigned integral type to represent wide characters, and use an
+ internal encoding of Unicode or UCS2. (See AIX, Microsoft NT,
+ Java, others.) Other systems, use a four byte, unsigned integral
+ type to represent wide characters, and use an internal encoding
+ of UCS4. (GNU/Linux systems using glibc, in particular.) The C
+ programming language (and thus C++) does not specify a specific
+ size for the type wchar_t.
+ </p><p>
+ Thus, portable C++ code cannot assume a byte size (or endianness) either.
+ </p></div><div class="section" title="Support for Unicode"><div class="titlepage"><div><div><h5 class="title"><a id="codecvt.design.unicode"/>Support for Unicode</h5></div></div></div><p>
+ Probably the most frequently asked question about code conversion
+ is: "So dudes, what's the deal with Unicode strings?"
+ The dude part is optional, but apparently the usefulness of
+ Unicode strings is pretty widely appreciated. Sadly, this specific
+ encoding (And other useful encodings like UTF8, UCS4, ISO 8859-10,
+ etc etc etc) are not mentioned in the C++ standard.
+ </p><p>
+ A couple of comments:
+ </p><p>
+ The thought that all one needs to convert between two arbitrary
+ codesets is two types and some kind of state argument is
+ unfortunate. In particular, encodings may be stateless. The naming
+ of the third parameter as stateT is unfortunate, as what is really
+ needed is some kind of generalized type that accounts for the
+ issues that abstract encodings will need. The minimum information
+ that is required includes:
+ </p><div class="itemizedlist"><ul class="itemizedlist"><li class="listitem"><p>
+ Identifiers for each of the codesets involved in the
+ conversion. For example, using the iconv family of functions
+ from the Single Unix Specification (what used to be called
+ X/Open) hosted on the GNU/Linux operating system allows
+ bi-directional mapping between far more than the following
+ tantalizing possibilities:
+ </p><p>
+ (An edited list taken from <code class="code">`iconv --list`</code> on a
+ Red Hat 6.2/Intel system:
+ </p><div class="blockquote"><blockquote class="blockquote"><pre class="programlisting">
+8859_1, 8859_9, 10646-1:1993, 10646-1:1993/UCS4, ARABIC, ARABIC7,
+ASCII, EUC-CN, EUC-JP, EUC-KR, EUC-TW, GREEK-CCIcode, GREEK, GREEK7-OLD,
+GREEK7, GREEK8, HEBREW, ISO-8859-1, ISO-8859-2, ISO-8859-3,
+ISO-8859-4, ISO-8859-5, ISO-8859-6, ISO-8859-7, ISO-8859-8,
+ISO-8859-9, ISO-8859-10, ISO-8859-11, ISO-8859-13, ISO-8859-14,
+ISO-8859-15, ISO-10646, ISO-10646/UCS2, ISO-10646/UCS4,
+ISO-10646/UTF-8, ISO-10646/UTF8, SHIFT-JIS, SHIFT_JIS, UCS-2, UCS-4,
+UCS2, UCS4, UNICODE, UNICODEBIG, UNICODELIcodeLE, US-ASCII, US, UTF-8,
+UTF-16, UTF8, UTF16).
+</pre></blockquote></div><p>
+For iconv-based implementations, string literals for each of the
+encodings (i.e. "UCS-2" and "UTF-8") are necessary,
+although for other,
+non-iconv implementations a table of enumerated values or some other
+mechanism may be required.
+</p></li><li class="listitem"><p>
+ Maximum length of the identifying string literal.
+</p></li><li class="listitem"><p>
+ Some encodings require explicit endian-ness. As such, some kind
+ of endian marker or other byte-order marker will be necessary. See
+ "Footnotes for C/C++ developers" in Haible for more information on
+ UCS-2/Unicode endian issues. (Summary: big endian seems most likely,
+ however implementations, most notably Microsoft, vary.)
+</p></li><li class="listitem"><p>
+ Types representing the conversion state, for conversions involving
+ the machinery in the "C" library, or the conversion descriptor, for
+ conversions using iconv (such as the type iconv_t.) Note that the
+ conversion descriptor encodes more information than a simple encoding
+ state type.
+</p></li><li class="listitem"><p>
+ Conversion descriptors for both directions of encoding. (i.e., both
+ UCS-2 to UTF-8 and UTF-8 to UCS-2.)
+</p></li><li class="listitem"><p>
+ Something to indicate if the conversion requested if valid.
+</p></li><li class="listitem"><p>
+ Something to represent if the conversion descriptors are valid.
+</p></li><li class="listitem"><p>
+ Some way to enforce strict type checking on the internal and
+ external types. As part of this, the size of the internal and
+ external types will need to be known.
+</p></li></ul></div></div><div class="section" title="Other Issues"><div class="titlepage"><div><div><h5 class="title"><a id="codecvt.design.issues"/>Other Issues</h5></div></div></div><p>
+In addition, multi-threaded and multi-locale environments also impact
+the design and requirements for code conversions. In particular, they
+affect the required specialization codecvt<wchar_t, char, mbstate_t>
+when implemented using standard "C" functions.
+</p><p>
+Three problems arise, one big, one of medium importance, and one small.
+</p><p>
+First, the small: mcsrtombs and wcsrtombs may not be multithread-safe
+on all systems required by the GNU tools. For GNU/Linux and glibc,
+this is not an issue.
+</p><p>
+Of medium concern, in the grand scope of things, is that the functions
+used to implement this specialization work on null-terminated
+strings. Buffers, especially file buffers, may not be null-terminated,
+thus giving conversions that end prematurely or are otherwise
+incorrect. Yikes!
+</p><p>
+The last, and fundamental problem, is the assumption of a global
+locale for all the "C" functions referenced above. For something like
+C++ iostreams (where codecvt is explicitly used) the notion of
+multiple locales is fundamental. In practice, most users may not run
+into this limitation. However, as a quality of implementation issue,
+the GNU C++ library would like to offer a solution that allows
+multiple locales and or simultaneous usage with computationally
+correct results. In short, libstdc++ is trying to offer, as an
+option, a high-quality implementation, damn the additional complexity!
+</p><p>
+For the required specialization codecvt<wchar_t, char, mbstate_t> ,
+conversions are made between the internal character set (always UCS4
+on GNU/Linux) and whatever the currently selected locale for the
+LC_CTYPE category implements.
+</p></div></div><div class="section" title="Implementation"><div class="titlepage"><div><div><h4 class="title"><a id="facet.codecvt.impl"/>Implementation</h4></div></div></div><p>
+The two required specializations are implemented as follows:
+</p><p>
+<code class="code">
+codecvt<char, char, mbstate_t>
+</code>
+</p><p>
+This is a degenerate (i.e., does nothing) specialization. Implementing
+this was a piece of cake.
+</p><p>
+<code class="code">
+codecvt<char, wchar_t, mbstate_t>
+</code>
+</p><p>
+This specialization, by specifying all the template parameters, pretty
+much ties the hands of implementors. As such, the implementation is
+straightforward, involving mcsrtombs for the conversions between char
+to wchar_t and wcsrtombs for conversions between wchar_t and char.
+</p><p>
+Neither of these two required specializations deals with Unicode
+characters. As such, libstdc++ implements a partial specialization
+of the codecvt class with and iconv wrapper class, encoding_state as the
+third template parameter.
+</p><p>
+This implementation should be standards conformant. First of all, the
+standard explicitly points out that instantiations on the third
+template parameter, stateT, are the proper way to implement
+non-required conversions. Second of all, the standard says (in Chapter
+17) that partial specializations of required classes are a-ok. Third
+of all, the requirements for the stateT type elsewhere in the standard
+(see 21.1.2 traits typedefs) only indicate that this type be copy
+constructible.
+</p><p>
+As such, the type encoding_state is defined as a non-templatized, POD
+type to be used as the third type of a codecvt instantiation. This
+type is just a wrapper class for iconv, and provides an easy interface
+to iconv functionality.
+</p><p>
+There are two constructors for encoding_state:
+</p><p>
+<code class="code">
+encoding_state() : __in_desc(0), __out_desc(0)
+</code>
+</p><p>
+This default constructor sets the internal encoding to some default
+(currently UCS4) and the external encoding to whatever is returned by
+nl_langinfo(CODESET).
+</p><p>
+<code class="code">
+encoding_state(const char* __int, const char* __ext)
+</code>
+</p><p>
+This constructor takes as parameters string literals that indicate the
+desired internal and external encoding. There are no defaults for
+either argument.
+</p><p>
+One of the issues with iconv is that the string literals identifying
+conversions are not standardized. Because of this, the thought of
+mandating and or enforcing some set of pre-determined valid
+identifiers seems iffy: thus, a more practical (and non-migraine
+inducing) strategy was implemented: end-users can specify any string
+(subject to a pre-determined length qualifier, currently 32 bytes) for
+encodings. It is up to the user to make sure that these strings are
+valid on the target system.
+</p><p>
+<code class="code">
+void
+_M_init()
+</code>
+</p><p>
+Strangely enough, this member function attempts to open conversion
+descriptors for a given encoding_state object. If the conversion
+descriptors are not valid, the conversion descriptors returned will
+not be valid and the resulting calls to the codecvt conversion
+functions will return error.
+</p><p>
+<code class="code">
+bool
+_M_good()
+</code>
+</p><p>
+Provides a way to see if the given encoding_state object has been
+properly initialized. If the string literals describing the desired
+internal and external encoding are not valid, initialization will
+fail, and this will return false. If the internal and external
+encodings are valid, but iconv_open could not allocate conversion
+descriptors, this will also return false. Otherwise, the object is
+ready to convert and will return true.
+</p><p>
+<code class="code">
+encoding_state(const encoding_state&)
+</code>
+</p><p>
+As iconv allocates memory and sets up conversion descriptors, the copy
+constructor can only copy the member data pertaining to the internal
+and external code conversions, and not the conversion descriptors
+themselves.
+</p><p>
+Definitions for all the required codecvt member functions are provided
+for this specialization, and usage of codecvt<internal character type,
+external character type, encoding_state> is consistent with other
+codecvt usage.
+</p></div><div class="section" title="Use"><div class="titlepage"><div><div><h4 class="title"><a id="facet.codecvt.use"/>Use</h4></div></div></div><p>A conversions involving string literal.</p><pre class="programlisting">
+ typedef codecvt_base::result result;
+ typedef unsigned short unicode_t;
+ typedef unicode_t int_type;
+ typedef char ext_type;
+ typedef encoding_state state_type;
+ typedef codecvt<int_type, ext_type, state_type> unicode_codecvt;
+
+ const ext_type* e_lit = "black pearl jasmine tea";
+ int size = strlen(e_lit);
+ int_type i_lit_base[24] =
+ { 25088, 27648, 24832, 25344, 27392, 8192, 28672, 25856, 24832, 29184,
+ 27648, 8192, 27136, 24832, 29440, 27904, 26880, 28160, 25856, 8192, 29696,
+ 25856, 24832, 2560
+ };
+ const int_type* i_lit = i_lit_base;
+ const ext_type* efrom_next;
+ const int_type* ifrom_next;
+ ext_type* e_arr = new ext_type[size + 1];
+ ext_type* eto_next;
+ int_type* i_arr = new int_type[size + 1];
+ int_type* ito_next;
+
+ // construct a locale object with the specialized facet.
+ locale loc(locale::classic(), new unicode_codecvt);
+ // sanity check the constructed locale has the specialized facet.
+ VERIFY( has_facet<unicode_codecvt>(loc) );
+ const unicode_codecvt& cvt = use_facet<unicode_codecvt>(loc);
+ // convert between const char* and unicode strings
+ unicode_codecvt::state_type state01("UNICODE", "ISO_8859-1");
+ initialize_state(state01);
+ result r1 = cvt.in(state01, e_lit, e_lit + size, efrom_next,
+ i_arr, i_arr + size, ito_next);
+ VERIFY( r1 == codecvt_base::ok );
+ VERIFY( !int_traits::compare(i_arr, i_lit, size) );
+ VERIFY( efrom_next == e_lit + size );
+ VERIFY( ito_next == i_arr + size );
+</pre></div><div class="section" title="Future"><div class="titlepage"><div><div><h4 class="title"><a id="facet.codecvt.future"/>Future</h4></div></div></div><div class="itemizedlist"><ul class="itemizedlist"><li class="listitem"><p>
+ a. things that are sketchy, or remain unimplemented:
+ do_encoding, max_length and length member functions
+ are only weakly implemented. I have no idea how to do
+ this correctly, and in a generic manner. Nathan?
+</p></li><li class="listitem"><p>
+ b. conversions involving std::string
+ </p><div class="itemizedlist"><ul class="itemizedlist"><li class="listitem"><p>
+ how should operators != and == work for string of
+ different/same encoding?
+ </p></li><li class="listitem"><p>
+ what is equal? A byte by byte comparison or an
+ encoding then byte comparison?
+ </p></li><li class="listitem"><p>
+ conversions between narrow, wide, and unicode strings
+ </p></li></ul></div></li><li class="listitem"><p>
+ c. conversions involving std::filebuf and std::ostream
+</p><div class="itemizedlist"><ul class="itemizedlist"><li class="listitem"><p>
+ how to initialize the state object in a
+ standards-conformant manner?
+ </p></li><li class="listitem"><p>
+ how to synchronize the "C" and "C++"
+ conversion information?
+ </p></li><li class="listitem"><p>
+ wchar_t/char internal buffers and conversions between
+ internal/external buffers?
+ </p></li></ul></div></li></ul></div></div><div class="bibliography" title="Bibliography"><div class="titlepage"><div><div><h4 class="title"><a id="facet.codecvt.biblio"/>Bibliography</h4></div></div></div><div class="biblioentry"><a id="id610059"/><p><span class="citetitle"><em class="citetitle">
+ The GNU C Library
+ </em>. </span><span class="author"><span class="firstname">Roland</span> <span class="surname">McGrath</span>. </span><span class="author"><span class="firstname">Ulrich</span> <span class="surname">Drepper</span>. </span><span class="copyright">Copyright © 2007 FSF. </span><span class="pagenums">
+ Chapters 6 Character Set Handling and 7 Locales and Internationalization
+ . </span></p></div><div class="biblioentry"><a id="id610099"/><p><span class="citetitle"><em class="citetitle">
+ Correspondence
+ </em>. </span><span class="author"><span class="firstname">Ulrich</span> <span class="surname">Drepper</span>. </span><span class="copyright">Copyright © 2002 . </span></p></div><div class="biblioentry"><a id="id610125"/><p><span class="citetitle"><em class="citetitle">
+ ISO/IEC 14882:1998 Programming languages - C++
+ </em>. </span><span class="copyright">Copyright © 1998 ISO. </span></p></div><div class="biblioentry"><a id="id610144"/><p><span class="citetitle"><em class="citetitle">
+ ISO/IEC 9899:1999 Programming languages - C
+ </em>. </span><span class="copyright">Copyright © 1999 ISO. </span></p></div><div class="biblioentry" title="System Interface Definitions, Issue 7 (IEEE Std. 1003.1-2008)"><a id="id610163"/><p><span class="title"><em>
+ <a class="link" href="http://www.opengroup.org/austin">
+ System Interface Definitions, Issue 7 (IEEE Std. 1003.1-2008)
+ </a>
+ </em>. </span><span class="copyright">Copyright © 2008
+ The Open Group/The Institute of Electrical and Electronics
+ Engineers, Inc.
+ . </span></p></div><div class="biblioentry"><a id="id610190"/><p><span class="citetitle"><em class="citetitle">
+ The C++ Programming Language, Special Edition
+ </em>. </span><span class="author"><span class="firstname">Bjarne</span> <span class="surname">Stroustrup</span>. </span><span class="copyright">Copyright © 2000 Addison Wesley, Inc.. </span><span class="pagenums">Appendix D. </span><span class="publisher"><span class="publishername">
+ Addison Wesley
+ . </span></span></p></div><div class="biblioentry"><a id="id610228"/><p><span class="citetitle"><em class="citetitle">
+ Standard C++ IOStreams and Locales
+ </em>. </span><span class="subtitle">
+ Advanced Programmer's Guide and Reference
+ . </span><span class="author"><span class="firstname">Angelika</span> <span class="surname">Langer</span>. </span><span class="author"><span class="firstname">Klaus</span> <span class="surname">Kreft</span>. </span><span class="copyright">Copyright © 2000 Addison Wesley Longman, Inc.. </span><span class="publisher"><span class="publishername">
+ Addison Wesley Longman
+ . </span></span></p></div><div class="biblioentry" title="A brief description of Normative Addendum 1"><a id="id610275"/><p><span class="title"><em>
+ <a class="link" href="http://www.lysator.liu.se/c/na1.html">
+ A brief description of Normative Addendum 1
+ </a>
+ </em>. </span><span class="author"><span class="firstname">Clive</span> <span class="surname">Feather</span>. </span><span class="pagenums">Extended Character Sets. </span></p></div><div class="biblioentry" title="The Unicode HOWTO"><a id="id610302"/><p><span class="title"><em>
+ <a class="link" href="http://tldp.org/HOWTO/Unicode-HOWTO.html">
+ The Unicode HOWTO
+ </a>
+ </em>. </span><span class="author"><span class="firstname">Bruno</span> <span class="surname">Haible</span>. </span></p></div><div class="biblioentry" title="UTF-8 and Unicode FAQ for Unix/Linux"><a id="id610326"/><p><span class="title"><em>
+ <a class="link" href="http://www.cl.cam.ac.uk/~mgk25/unicode.html">
+ UTF-8 and Unicode FAQ for Unix/Linux
+ </a>
+ </em>. </span><span class="author"><span class="firstname">Markus</span> <span class="surname">Khun</span>. </span></p></div></div></div><div class="section" title="messages"><div class="titlepage"><div><div><h3 class="title"><a id="manual.localization.facet.messages"/>messages</h3></div></div></div><p>
+The std::messages facet implements message retrieval functionality
+equivalent to Java's java.text.MessageFormat .using either GNU gettext
+or IEEE 1003.1-200 functions.
+</p><div class="section" title="Requirements"><div class="titlepage"><div><div><h4 class="title"><a id="facet.messages.req"/>Requirements</h4></div></div></div><p>
+The std::messages facet is probably the most vaguely defined facet in
+the standard library. It's assumed that this facility was built into
+the standard library in order to convert string literals from one
+locale to the other. For instance, converting the "C" locale's
+<code class="code">const char* c = "please"</code> to a German-localized <code class="code">"bitte"</code>
+during program execution.
+</p><div class="blockquote"><blockquote class="blockquote"><p>
+22.2.7.1 - Template class messages [lib.locale.messages]
+</p></blockquote></div><p>
+This class has three public member functions, which directly
+correspond to three protected virtual member functions.
+</p><p>
+The public member functions are:
+</p><p>
+<code class="code">catalog open(const string&, const locale&) const</code>
+</p><p>
+<code class="code">string_type get(catalog, int, int, const string_type&) const</code>
+</p><p>
+<code class="code">void close(catalog) const</code>
+</p><p>
+While the virtual functions are:
+</p><p>
+<code class="code">catalog do_open(const string&, const locale&) const</code>
+</p><div class="blockquote"><blockquote class="blockquote"><p>
+<span class="emphasis"><em>
+-1- Returns: A value that may be passed to get() to retrieve a
+message, from the message catalog identified by the string name
+according to an implementation-defined mapping. The result can be used
+until it is passed to close(). Returns a value less than 0 if no such
+catalog can be opened.
+</em></span>
+</p></blockquote></div><p>
+<code class="code">string_type do_get(catalog, int, int, const string_type&) const</code>
+</p><div class="blockquote"><blockquote class="blockquote"><p>
+<span class="emphasis"><em>
+-3- Requires: A catalog cat obtained from open() and not yet closed.
+-4- Returns: A message identified by arguments set, msgid, and dfault,
+according to an implementation-defined mapping. If no such message can
+be found, returns dfault.
+</em></span>
+</p></blockquote></div><p>
+<code class="code">void do_close(catalog) const</code>
+</p><div class="blockquote"><blockquote class="blockquote"><p>
+<span class="emphasis"><em>
+-5- Requires: A catalog cat obtained from open() and not yet closed.
+-6- Effects: Releases unspecified resources associated with cat.
+-7- Notes: The limit on such resources, if any, is implementation-defined.
+</em></span>
+</p></blockquote></div></div><div class="section" title="Design"><div class="titlepage"><div><div><h4 class="title"><a id="facet.messages.design"/>Design</h4></div></div></div><p>
+A couple of notes on the standard.
+</p><p>
+First, why is <code class="code">messages_base::catalog</code> specified as a typedef
+to int? This makes sense for implementations that use
+<code class="code">catopen</code> and define <code class="code">nl_catd</code> as int, but not for
+others. Fortunately, it's not heavily used and so only a minor irritant.
+This has been reported as a possible defect in the standard (LWG 2028).
+</p><p>
+Second, by making the member functions <code class="code">const</code>, it is
+impossible to save state in them. Thus, storing away information used
+in the 'open' member function for use in 'get' is impossible. This is
+unfortunate.
+</p><p>
+The 'open' member function in particular seems to be oddly
+designed. The signature seems quite peculiar. Why specify a <code class="code">const
+string& </code> argument, for instance, instead of just <code class="code">const
+char*</code>? Or, why specify a <code class="code">const locale&</code> argument that is
+to be used in the 'get' member function? How, exactly, is this locale
+argument useful? What was the intent? It might make sense if a locale
+argument was associated with a given default message string in the
+'open' member function, for instance. Quite murky and unclear, on
+reflection.
+</p><p>
+Lastly, it seems odd that messages, which explicitly require code
+conversion, don't use the codecvt facet. Because the messages facet
+has only one template parameter, it is assumed that ctype, and not
+codecvt, is to be used to convert between character sets.
+</p><p>
+It is implicitly assumed that the locale for the default message
+string in 'get' is in the "C" locale. Thus, all source code is assumed
+to be written in English, so translations are always from "en_US" to
+other, explicitly named locales.
+</p></div><div class="section" title="Implementation"><div class="titlepage"><div><div><h4 class="title"><a id="facet.messages.impl"/>Implementation</h4></div></div></div><div class="section" title="Models"><div class="titlepage"><div><div><h5 class="title"><a id="messages.impl.models"/>Models</h5></div></div></div><p>
+ This is a relatively simple class, on the face of it. The standard
+ specifies very little in concrete terms, so generic
+ implementations that are conforming yet do very little are the
+ norm. Adding functionality that would be useful to programmers and
+ comparable to Java's java.text.MessageFormat takes a bit of work,
+ and is highly dependent on the capabilities of the underlying
+ operating system.
+ </p><p>
+ Three different mechanisms have been provided, selectable via
+ configure flags:
+ </p><div class="itemizedlist"><ul class="itemizedlist"><li class="listitem"><p>
+ generic
+ </p><p>
+ This model does very little, and is what is used by default.
+ </p></li><li class="listitem"><p>
+ gnu
+ </p><p>
+ The gnu model is complete and fully tested. It's based on the
+ GNU gettext package, which is part of glibc. It uses the
+ functions <code class="code">textdomain, bindtextdomain, gettext</code> to
+ implement full functionality. Creating message catalogs is a
+ relatively straight-forward process and is lightly documented
+ below, and fully documented in gettext's distributed
+ documentation.
+ </p></li><li class="listitem"><p>
+ ieee_1003.1-200x
+ </p><p>
+ This is a complete, though untested, implementation based on
+ the IEEE standard. The functions <code class="code">catopen, catgets,
+ catclose</code> are used to retrieve locale-specific messages
+ given the appropriate message catalogs that have been
+ constructed for their use. Note, the script <code class="code">
+ po2msg.sed</code> that is part of the gettext distribution can
+ convert gettext catalogs into catalogs that
+ <code class="code">catopen</code> can use.
+ </p></li></ul></div><p>
+A new, standards-conformant non-virtual member function signature was
+added for 'open' so that a directory could be specified with a given
+message catalog. This simplifies calling conventions for the gnu
+model.
+</p></div><div class="section" title="The GNU Model"><div class="titlepage"><div><div><h5 class="title"><a id="messages.impl.gnu"/>The GNU Model</h5></div></div></div><p>
+ The messages facet, because it is retrieving and converting
+ between characters sets, depends on the ctype and perhaps the
+ codecvt facet in a given locale. In addition, underlying "C"
+ library locale support is necessary for more than just the
+ <code class="code">LC_MESSAGES</code> mask: <code class="code">LC_CTYPE</code> is also
+ necessary. To avoid any unpleasantness, all bits of the "C" mask
+ (i.e. <code class="code">LC_ALL</code>) are set before retrieving messages.
+ </p><p>
+ Making the message catalogs can be initially tricky, but become
+ quite simple with practice. For complete info, see the gettext
+ documentation. Here's an idea of what is required:
+ </p><div class="itemizedlist"><ul class="itemizedlist"><li class="listitem"><p>
+ Make a source file with the required string literals that need
+ to be translated. See <code class="code">intl/string_literals.cc</code> for
+ an example.
+ </p></li><li class="listitem"><p>
+ Make initial catalog (see "4 Making the PO Template File" from
+ the gettext docs).</p><p>
+ <code class="code"> xgettext --c++ --debug string_literals.cc -o libstdc++.pot </code>
+ </p></li><li class="listitem"><p>Make language and country-specific locale catalogs.</p><p>
+ <code class="code">cp libstdc++.pot fr_FR.po</code>
+ </p><p>
+ <code class="code">cp libstdc++.pot de_DE.po</code>
+ </p></li><li class="listitem"><p>
+ Edit localized catalogs in emacs so that strings are
+ translated.
+ </p><p>
+ <code class="code">emacs fr_FR.po</code>
+ </p></li><li class="listitem"><p>Make the binary mo files.</p><p>
+ <code class="code">msgfmt fr_FR.po -o fr_FR.mo</code>
+ </p><p>
+ <code class="code">msgfmt de_DE.po -o de_DE.mo</code>
+ </p></li><li class="listitem"><p>Copy the binary files into the correct directory structure.</p><p>
+ <code class="code">cp fr_FR.mo (dir)/fr_FR/LC_MESSAGES/libstdc++.mo</code>
+ </p><p>
+ <code class="code">cp de_DE.mo (dir)/de_DE/LC_MESSAGES/libstdc++.mo</code>
+ </p></li><li class="listitem"><p>Use the new message catalogs.</p><p>
+ <code class="code">locale loc_de("de_DE");</code>
+ </p><p>
+ <code class="code">
+ use_facet<messages<char> >(loc_de).open("libstdc++", locale(), dir);
+ </code>
+ </p></li></ul></div></div></div><div class="section" title="Use"><div class="titlepage"><div><div><h4 class="title"><a id="facet.messages.use"/>Use</h4></div></div></div><p>
+ A simple example using the GNU model of message conversion.
+ </p><pre class="programlisting">
+#include <iostream>
+#include <locale>
+using namespace std;
+
+void test01()
+{
+ typedef messages<char>::catalog catalog;
+ const char* dir =
+ "/mnt/egcs/build/i686-pc-linux-gnu/libstdc++/po/share/locale";
+ const locale loc_de("de_DE");
+ const messages<char>& mssg_de = use_facet<messages<char> >(loc_de);
+
+ catalog cat_de = mssg_de.open("libstdc++", loc_de, dir);
+ string s01 = mssg_de.get(cat_de, 0, 0, "please");
+ string s02 = mssg_de.get(cat_de, 0, 0, "thank you");
+ cout << "please in german:" << s01 << '\n';
+ cout << "thank you in german:" << s02 << '\n';
+ mssg_de.close(cat_de);
+}
+</pre></div><div class="section" title="Future"><div class="titlepage"><div><div><h4 class="title"><a id="facet.messages.future"/>Future</h4></div></div></div><div class="itemizedlist"><ul class="itemizedlist"><li class="listitem"><p>
+ Things that are sketchy, or remain unimplemented:
+ </p><div class="itemizedlist"><ul class="itemizedlist"><li class="listitem"><p>
+ _M_convert_from_char, _M_convert_to_char are in flux,
+ depending on how the library ends up doing character set
+ conversions. It might not be possible to do a real character
+ set based conversion, due to the fact that the template
+ parameter for messages is not enough to instantiate the
+ codecvt facet (1 supplied, need at least 2 but would prefer
+ 3).
+ </p></li><li class="listitem"><p>
+ There are issues with gettext needing the global locale set
+ to extract a message. This dependence on the global locale
+ makes the current "gnu" model non MT-safe. Future versions
+ of glibc, i.e. glibc 2.3.x will fix this, and the C++ library
+ bits are already in place.
+ </p></li></ul></div></li><li class="listitem"><p>
+ Development versions of the GNU "C" library, glibc 2.3 will allow
+ a more efficient, MT implementation of std::messages, and will
+ allow the removal of the _M_name_messages data member. If this is
+ done, it will change the library ABI. The C++ parts to support
+ glibc 2.3 have already been coded, but are not in use: once this
+ version of the "C" library is released, the marked parts of the
+ messages implementation can be switched over to the new "C"
+ library functionality.
+ </p></li><li class="listitem"><p>
+ At some point in the near future, std::numpunct will probably use
+ std::messages facilities to implement truename/falsename
+ correctly. This is currently not done, but entries in
+ libstdc++.pot have already been made for "true" and "false" string
+ literals, so all that remains is the std::numpunct coding and the
+ configure/make hassles to make the installed library search its
+ own catalog. Currently the libstdc++.mo catalog is only searched
+ for the testsuite cases involving messages members.
+ </p></li><li class="listitem"><p> The following member functions:</p><p>
+ <code class="code">
+ catalog
+ open(const basic_string<char>& __s, const locale& __loc) const
+ </code>
+ </p><p>
+ <code class="code">
+ catalog
+ open(const basic_string<char>&, const locale&, const char*) const;
+ </code>
+ </p><p>
+ Don't actually return a "value less than 0 if no such catalog
+ can be opened" as required by the standard in the "gnu"
+ model. As of this writing, it is unknown how to query to see
+ if a specified message catalog exists using the gettext
+ package.
+ </p></li></ul></div></div><div class="bibliography" title="Bibliography"><div class="titlepage"><div><div><h4 class="title"><a id="facet.messages.biblio"/>Bibliography</h4></div></div></div><div class="biblioentry"><a id="id611002"/><p><span class="citetitle"><em class="citetitle">
+ The GNU C Library
+ </em>. </span><span class="author"><span class="firstname">Roland</span> <span class="surname">McGrath</span>. </span><span class="author"><span class="firstname">Ulrich</span> <span class="surname">Drepper</span>. </span><span class="copyright">Copyright © 2007 FSF. </span><span class="pagenums">Chapters 6 Character Set Handling, and 7 Locales and Internationalization
+ . </span></p></div><div class="biblioentry"><a id="id611041"/><p><span class="citetitle"><em class="citetitle">
+ Correspondence
+ </em>. </span><span class="author"><span class="firstname">Ulrich</span> <span class="surname">Drepper</span>. </span><span class="copyright">Copyright © 2002 . </span></p></div><div class="biblioentry"><a id="id611067"/><p><span class="citetitle"><em class="citetitle">
+ ISO/IEC 14882:1998 Programming languages - C++
+ </em>. </span><span class="copyright">Copyright © 1998 ISO. </span></p></div><div class="biblioentry"><a id="id611086"/><p><span class="citetitle"><em class="citetitle">
+ ISO/IEC 9899:1999 Programming languages - C
+ </em>. </span><span class="copyright">Copyright © 1999 ISO. </span></p></div><div class="biblioentry" title="System Interface Definitions, Issue 7 (IEEE Std. 1003.1-2008)"><a id="id611105"/><p><span class="title"><em>
+ <a class="link" href="http://www.opengroup.org/austin">
+ System Interface Definitions, Issue 7 (IEEE Std. 1003.1-2008)
+ </a>
+ </em>. </span><span class="copyright">Copyright © 2008
+ The Open Group/The Institute of Electrical and Electronics
+ Engineers, Inc.
+ . </span></p></div><div class="biblioentry"><a id="id611132"/><p><span class="citetitle"><em class="citetitle">
+ The C++ Programming Language, Special Edition
+ </em>. </span><span class="author"><span class="firstname">Bjarne</span> <span class="surname">Stroustrup</span>. </span><span class="copyright">Copyright © 2000 Addison Wesley, Inc.. </span><span class="pagenums">Appendix D. </span><span class="publisher"><span class="publishername">
+ Addison Wesley
+ . </span></span></p></div><div class="biblioentry"><a id="id611171"/><p><span class="citetitle"><em class="citetitle">
+ Standard C++ IOStreams and Locales
+ </em>. </span><span class="subtitle">
+ Advanced Programmer's Guide and Reference
+ . </span><span class="author"><span class="firstname">Angelika</span> <span class="surname">Langer</span>. </span><span class="author"><span class="firstname">Klaus</span> <span class="surname">Kreft</span>. </span><span class="copyright">Copyright © 2000 Addison Wesley Longman, Inc.. </span><span class="publisher"><span class="publishername">
+ Addison Wesley Longman
+ . </span></span></p></div><div class="biblioentry" title="API Specifications, Java Platform"><a id="id611218"/><p><span class="title"><em>
+ <a class="link" href="http://java.sun.com/reference/api/index.html">
+ API Specifications, Java Platform
+ </a>
+ </em>. </span><span class="pagenums">java.util.Properties, java.text.MessageFormat,
+java.util.Locale, java.util.ResourceBundle
+ . </span></p></div><div class="biblioentry" title="GNU gettext tools, version 0.10.38, Native Language Support Library and Tools."><a id="id611237"/><p><span class="title"><em>
+ <a class="link" href="http://www.gnu.org/software/gettext">
+ GNU gettext tools, version 0.10.38, Native Language Support
+ Library and Tools.
+ </a>
+ </em>. </span></p></div></div></div></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td align="left"><a accesskey="p" href="localization.html">Prev</a> </td><td align="center"><a accesskey="u" href="localization.html">Up</a></td><td align="right"> <a accesskey="n" href="containers.html">Next</a></td></tr><tr><td align="left" valign="top">Chapter 8.
+ Localization
+
+ </td><td align="center"><a accesskey="h" href="../index.html">Home</a></td><td align="right" valign="top"> Chapter 9.
+ Containers
+
+</td></tr></table></div></body></html>
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