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| <html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>codecvt</title><meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /><meta name="keywords" content=" ISO C++ , codecvt " /><meta name="keywords" content=" ISO C++ , library " /><link rel="start" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="bk01pt06ch15.html" title="Chapter 15. Facets aka Categories" /><link rel="prev" href="bk01pt06ch15.html" title="Chapter 15. 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="bk01pt06ch15.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="id394270"></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="id464999"></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="id465028"></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="id426183"></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="id426202"></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="id402703"></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="id413189"></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="id427920"></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="id458138"></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="id458167"></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="bk01pt06ch15.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="bk01pt06ch15.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. Facets aka Categories </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> messages</td></tr></table></div></body></html> |