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Facets aka Categories" /><link rel="next" href="messages.html" title="messages" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">codecvt</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="facets.html">Prev</a> </td><th width="60%" align="center">Chapter 15. Facets aka Categories</th><td width="20%" align="right"> <a accesskey="n" href="messages.html">Next</a></td></tr></table><hr /></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.codecvt"></a>codecvt</h2></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">C</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="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="facet.codecvt.req"></a>Requirements</h3></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="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="facet.codecvt.design"></a>Design</h3></div></div></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="codecvt.design.wchar_t_size"></a><span class="type">wchar_t</span> Size</h4></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="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="codecvt.design.unicode"></a>Support for Unicode</h4></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 type="disc"><li><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><p> - Maximum length of the identifying string literal. -</p></li><li><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><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><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><p> - Something to indicate if the conversion requested if valid. -</p></li><li><p> - Something to represent if the conversion descriptors are valid. -</p></li><li><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="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="codecvt.design.issues"></a>Other Issues</h4></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="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="facet.codecvt.impl"></a>Implementation</h3></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="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="facet.codecvt.use"></a>Use</h3></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="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="facet.codecvt.future"></a>Future</h3></div></div></div><div class="itemizedlist"><ul type="disc"><li><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><p> - b. conversions involving std::string - </p><div class="itemizedlist"><ul type="circle"><li><p> - how should operators != and == work for string of - different/same encoding? - </p></li><li><p> - what is equal? A byte by byte comparison or an - encoding then byte comparison? - </p></li><li><p> - conversions between narrow, wide, and unicode strings - </p></li></ul></div></li><li><p> - c. conversions involving std::filebuf and std::ostream -</p><div class="itemizedlist"><ul type="circle"><li><p> - how to initialize the state object in a - standards-conformant manner? - </p></li><li><p> - how to synchronize the "C" and "C++" - conversion information? - </p></li><li><p> - wchar_t/char internal buffers and conversions between - internal/external buffers? - </p></li></ul></div></li></ul></div></div><div class="bibliography"><div class="titlepage"><div><div><h3 class="title"><a id="facet.codecvt.biblio"></a>Bibliography</h3></div></div></div><div class="biblioentry"><a id="id415012"></a><p><span class="title"><i> - 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="id514935"></a><p><span class="title"><i> - 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="id416284"></a><p><span class="title"><i> - ISO/IEC 14882:1998 Programming languages - C++ - </i>. </span><span class="copyright">Copyright © 1998 ISO. </span></p></div><div class="biblioentry"><a id="id416302"></a><p><span class="title"><i> - ISO/IEC 9899:1999 Programming languages - C - </i>. </span><span class="copyright">Copyright © 1999 ISO. </span></p></div><div class="biblioentry"><a id="id459727"></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="id459755"></a><p><span class="title"><i> - 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"> - Addison Wesley - . </span></span></p></div><div class="biblioentry"><a id="id430786"></a><p><span class="title"><i> - Standard C++ IOStreams and Locales - </i>. </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"><a id="id407191"></a><p><span class="title"><i> - A brief description of Normative Addendum 1 - </i>. </span><span class="author"><span class="firstname">Clive</span> <span class="surname">Feather</span>. </span><span class="pagenums">Extended Character Sets. </span><span class="biblioid"> - <a class="ulink" href="http://www.lysator.liu.se/c/na1.html" target="_top"> - </a> - . </span></p></div><div class="biblioentry"><a id="id394112"></a><p><span class="title"><i> - The Unicode HOWTO - </i>. </span><span class="author"><span class="firstname">Bruno</span> <span class="surname">Haible</span>. </span><span class="biblioid"> - <a class="ulink" href="ftp://ftp.ilog.fr/pub/Users/haible/utf8/Unicode-HOWTO.html" target="_top"> - </a> - . </span></p></div><div class="biblioentry"><a id="id394140"></a><p><span class="title"><i> - UTF-8 and Unicode FAQ for Unix/Linux - </i>. </span><span class="author"><span class="firstname">Markus</span> <span class="surname">Khun</span>. </span><span class="biblioid"> - <a class="ulink" href="http://www.cl.cam.ac.uk/~mgk25/unicode.html" target="_top"> - </a> - . </span></p></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="facets.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="facets.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="messages.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 15. 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