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2009-03-17 Benjamin Kosnik <bkoz@redhat.com> * doc/xml/manual/appendix_contributing.xml: Add docbook style sheet version information. Table-ize docbook element examples. * doc/xml/manual/using.xml: Human-readable header markup. Alphabetized. Add new headers. * doc/html: Regenerate. From-SVN: r144923
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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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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 V1.74.0" /><meta name="keywords" content=" ISO C++ , codecvt " /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" 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>
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The standard class codecvt attempts to address conversions between
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different character encoding schemes. In particular, the standard
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attempts to detail conversions between the implementation-defined wide
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characters (hereafter referred to as wchar_t) and the standard type
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char that is so beloved in classic “<span class="quote">C</span>” (which can now be
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referred to as narrow characters.) This document attempts to describe
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how the GNU libstdc++ implementation deals with the conversion between
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wide and narrow characters, and also presents a framework for dealing
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with the huge number of other encodings that iconv can convert,
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including Unicode and UTF8. Design issues and requirements are
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addressed, and examples of correct usage for both the required
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specializations for wide and narrow characters and the
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implementation-provided extended functionality are given.
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</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>
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Around page 425 of the C++ Standard, this charming heading comes into view:
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</p><div class="blockquote"><blockquote class="blockquote"><p>
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22.2.1.5 - Template class codecvt
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</p></blockquote></div><p>
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The text around the codecvt definition gives some clues:
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</p><div class="blockquote"><blockquote class="blockquote"><p>
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<span class="emphasis"><em>
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-1- The class codecvt<internT,externT,stateT> is for use when
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converting from one codeset to another, such as from wide characters
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to multibyte characters, between wide character encodings such as
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Unicode and EUC.
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</em></span>
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</p></blockquote></div><p>
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Hmm. So, in some unspecified way, Unicode encodings and
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translations between other character sets should be handled by this
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class.
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</p><div class="blockquote"><blockquote class="blockquote"><p>
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<span class="emphasis"><em>
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-2- The stateT argument selects the pair of codesets being mapped between.
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</em></span>
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</p></blockquote></div><p>
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Ah ha! Another clue...
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</p><div class="blockquote"><blockquote class="blockquote"><p>
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<span class="emphasis"><em>
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-3- The instantiations required in the Table ??
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(lib.locale.category), namely codecvt<wchar_t,char,mbstate_t> and
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codecvt<char,char,mbstate_t>, convert the implementation-defined
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native character set. codecvt<char,char,mbstate_t> implements a
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degenerate conversion; it does not convert at
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all. codecvt<wchar_t,char,mbstate_t> converts between the native
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character sets for tiny and wide characters. Instantiations on
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mbstate_t perform conversion between encodings known to the library
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implementor. Other encodings can be converted by specializing on a
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user-defined stateT type. The stateT object can contain any state that
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is useful to communicate to or from the specialized do_convert member.
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</em></span>
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</p></blockquote></div><p>
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At this point, a couple points become clear:
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</p><p>
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One: The standard clearly implies that attempts to add non-required
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(yet useful and widely used) conversions need to do so through the
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third template parameter, stateT.</p><p>
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Two: The required conversions, by specifying mbstate_t as the third
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template parameter, imply an implementation strategy that is mostly
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(or wholly) based on the underlying C library, and the functions
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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>
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The simple implementation detail of wchar_t's size seems to
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repeatedly confound people. Many systems use a two byte,
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unsigned integral type to represent wide characters, and use an
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internal encoding of Unicode or UCS2. (See AIX, Microsoft NT,
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Java, others.) Other systems, use a four byte, unsigned integral
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type to represent wide characters, and use an internal encoding
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of UCS4. (GNU/Linux systems using glibc, in particular.) The C
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programming language (and thus C++) does not specify a specific
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size for the type wchar_t.
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</p><p>
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Thus, portable C++ code cannot assume a byte size (or endianness) either.
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</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>
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Probably the most frequently asked question about code conversion
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is: "So dudes, what's the deal with Unicode strings?"
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The dude part is optional, but apparently the usefulness of
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Unicode strings is pretty widely appreciated. Sadly, this specific
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encoding (And other useful encodings like UTF8, UCS4, ISO 8859-10,
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etc etc etc) are not mentioned in the C++ standard.
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</p><p>
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A couple of comments:
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</p><p>
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The thought that all one needs to convert between two arbitrary
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codesets is two types and some kind of state argument is
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unfortunate. In particular, encodings may be stateless. The naming
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of the third parameter as stateT is unfortunate, as what is really
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needed is some kind of generalized type that accounts for the
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issues that abstract encodings will need. The minimum information
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that is required includes:
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</p><div class="itemizedlist"><ul type="disc"><li><p>
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Identifiers for each of the codesets involved in the
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conversion. For example, using the iconv family of functions
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from the Single Unix Specification (what used to be called
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X/Open) hosted on the GNU/Linux operating system allows
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bi-directional mapping between far more than the following
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tantalizing possibilities:
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</p><p>
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(An edited list taken from <code class="code">`iconv --list`</code> on a
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Red Hat 6.2/Intel system:
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</p><div class="blockquote"><blockquote class="blockquote"><pre class="programlisting">
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8859_1, 8859_9, 10646-1:1993, 10646-1:1993/UCS4, ARABIC, ARABIC7,
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ASCII, EUC-CN, EUC-JP, EUC-KR, EUC-TW, GREEK-CCIcode, GREEK, GREEK7-OLD,
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GREEK7, GREEK8, HEBREW, ISO-8859-1, ISO-8859-2, ISO-8859-3,
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ISO-8859-4, ISO-8859-5, ISO-8859-6, ISO-8859-7, ISO-8859-8,
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ISO-8859-9, ISO-8859-10, ISO-8859-11, ISO-8859-13, ISO-8859-14,
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ISO-8859-15, ISO-10646, ISO-10646/UCS2, ISO-10646/UCS4,
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ISO-10646/UTF-8, ISO-10646/UTF8, SHIFT-JIS, SHIFT_JIS, UCS-2, UCS-4,
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UCS2, UCS4, UNICODE, UNICODEBIG, UNICODELIcodeLE, US-ASCII, US, UTF-8,
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UTF-16, UTF8, UTF16).
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</pre></blockquote></div><p>
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For iconv-based implementations, string literals for each of the
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encodings (i.e. "UCS-2" and "UTF-8") are necessary,
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although for other,
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non-iconv implementations a table of enumerated values or some other
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mechanism may be required.
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</p></li><li><p>
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Maximum length of the identifying string literal.
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</p></li><li><p>
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Some encodings require explicit endian-ness. As such, some kind
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of endian marker or other byte-order marker will be necessary. See
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"Footnotes for C/C++ developers" in Haible for more information on
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UCS-2/Unicode endian issues. (Summary: big endian seems most likely,
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however implementations, most notably Microsoft, vary.)
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</p></li><li><p>
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Types representing the conversion state, for conversions involving
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the machinery in the "C" library, or the conversion descriptor, for
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conversions using iconv (such as the type iconv_t.) Note that the
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conversion descriptor encodes more information than a simple encoding
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state type.
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</p></li><li><p>
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Conversion descriptors for both directions of encoding. (i.e., both
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UCS-2 to UTF-8 and UTF-8 to UCS-2.)
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</p></li><li><p>
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Something to indicate if the conversion requested if valid.
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</p></li><li><p>
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Something to represent if the conversion descriptors are valid.
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</p></li><li><p>
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Some way to enforce strict type checking on the internal and
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external types. As part of this, the size of the internal and
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external types will need to be known.
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</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>
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In addition, multi-threaded and multi-locale environments also impact
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the design and requirements for code conversions. In particular, they
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affect the required specialization codecvt<wchar_t, char, mbstate_t>
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when implemented using standard "C" functions.
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</p><p>
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Three problems arise, one big, one of medium importance, and one small.
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</p><p>
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First, the small: mcsrtombs and wcsrtombs may not be multithread-safe
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on all systems required by the GNU tools. For GNU/Linux and glibc,
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this is not an issue.
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</p><p>
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Of medium concern, in the grand scope of things, is that the functions
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used to implement this specialization work on null-terminated
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strings. Buffers, especially file buffers, may not be null-terminated,
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thus giving conversions that end prematurely or are otherwise
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incorrect. Yikes!
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</p><p>
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The last, and fundamental problem, is the assumption of a global
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locale for all the "C" functions referenced above. For something like
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C++ iostreams (where codecvt is explicitly used) the notion of
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multiple locales is fundamental. In practice, most users may not run
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into this limitation. However, as a quality of implementation issue,
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the GNU C++ library would like to offer a solution that allows
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multiple locales and or simultaneous usage with computationally
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correct results. In short, libstdc++ is trying to offer, as an
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option, a high-quality implementation, damn the additional complexity!
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</p><p>
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For the required specialization codecvt<wchar_t, char, mbstate_t> ,
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conversions are made between the internal character set (always UCS4
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on GNU/Linux) and whatever the currently selected locale for the
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LC_CTYPE category implements.
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</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>
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The two required specializations are implemented as follows:
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</p><p>
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<code class="code">
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codecvt<char, char, mbstate_t>
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</code>
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</p><p>
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This is a degenerate (i.e., does nothing) specialization. Implementing
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this was a piece of cake.
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</p><p>
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<code class="code">
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codecvt<char, wchar_t, mbstate_t>
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</code>
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</p><p>
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This specialization, by specifying all the template parameters, pretty
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much ties the hands of implementors. As such, the implementation is
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straightforward, involving mcsrtombs for the conversions between char
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to wchar_t and wcsrtombs for conversions between wchar_t and char.
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</p><p>
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Neither of these two required specializations deals with Unicode
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characters. As such, libstdc++ implements a partial specialization
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of the codecvt class with and iconv wrapper class, encoding_state as the
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third template parameter.
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</p><p>
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This implementation should be standards conformant. First of all, the
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standard explicitly points out that instantiations on the third
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template parameter, stateT, are the proper way to implement
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non-required conversions. Second of all, the standard says (in Chapter
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17) that partial specializations of required classes are a-ok. Third
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of all, the requirements for the stateT type elsewhere in the standard
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(see 21.1.2 traits typedefs) only indicate that this type be copy
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constructible.
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</p><p>
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As such, the type encoding_state is defined as a non-templatized, POD
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type to be used as the third type of a codecvt instantiation. This
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type is just a wrapper class for iconv, and provides an easy interface
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to iconv functionality.
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</p><p>
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There are two constructors for encoding_state:
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</p><p>
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<code class="code">
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encoding_state() : __in_desc(0), __out_desc(0)
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</code>
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</p><p>
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This default constructor sets the internal encoding to some default
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(currently UCS4) and the external encoding to whatever is returned by
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nl_langinfo(CODESET).
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</p><p>
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<code class="code">
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encoding_state(const char* __int, const char* __ext)
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</code>
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</p><p>
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This constructor takes as parameters string literals that indicate the
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desired internal and external encoding. There are no defaults for
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either argument.
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</p><p>
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One of the issues with iconv is that the string literals identifying
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conversions are not standardized. Because of this, the thought of
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mandating and or enforcing some set of pre-determined valid
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identifiers seems iffy: thus, a more practical (and non-migraine
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inducing) strategy was implemented: end-users can specify any string
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(subject to a pre-determined length qualifier, currently 32 bytes) for
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encodings. It is up to the user to make sure that these strings are
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valid on the target system.
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</p><p>
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<code class="code">
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void
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_M_init()
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</code>
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</p><p>
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Strangely enough, this member function attempts to open conversion
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descriptors for a given encoding_state object. If the conversion
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descriptors are not valid, the conversion descriptors returned will
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not be valid and the resulting calls to the codecvt conversion
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functions will return error.
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</p><p>
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<code class="code">
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bool
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_M_good()
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</code>
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</p><p>
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Provides a way to see if the given encoding_state object has been
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properly initialized. If the string literals describing the desired
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internal and external encoding are not valid, initialization will
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fail, and this will return false. If the internal and external
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encodings are valid, but iconv_open could not allocate conversion
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descriptors, this will also return false. Otherwise, the object is
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ready to convert and will return true.
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</p><p>
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<code class="code">
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encoding_state(const encoding_state&)
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</code>
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</p><p>
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As iconv allocates memory and sets up conversion descriptors, the copy
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constructor can only copy the member data pertaining to the internal
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and external code conversions, and not the conversion descriptors
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themselves.
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</p><p>
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Definitions for all the required codecvt member functions are provided
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for this specialization, and usage of codecvt<internal character type,
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external character type, encoding_state> is consistent with other
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codecvt usage.
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</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">
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typedef codecvt_base::result result;
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typedef unsigned short unicode_t;
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typedef unicode_t int_type;
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typedef char ext_type;
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typedef encoding_state state_type;
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typedef codecvt<int_type, ext_type, state_type> unicode_codecvt;
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const ext_type* e_lit = "black pearl jasmine tea";
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int size = strlen(e_lit);
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int_type i_lit_base[24] =
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{ 25088, 27648, 24832, 25344, 27392, 8192, 28672, 25856, 24832, 29184,
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27648, 8192, 27136, 24832, 29440, 27904, 26880, 28160, 25856, 8192, 29696,
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25856, 24832, 2560
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};
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const int_type* i_lit = i_lit_base;
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const ext_type* efrom_next;
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const int_type* ifrom_next;
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ext_type* e_arr = new ext_type[size + 1];
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ext_type* eto_next;
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int_type* i_arr = new int_type[size + 1];
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int_type* ito_next;
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// construct a locale object with the specialized facet.
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locale loc(locale::classic(), new unicode_codecvt);
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// sanity check the constructed locale has the specialized facet.
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VERIFY( has_facet<unicode_codecvt>(loc) );
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const unicode_codecvt& cvt = use_facet<unicode_codecvt>(loc);
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// convert between const char* and unicode strings
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unicode_codecvt::state_type state01("UNICODE", "ISO_8859-1");
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initialize_state(state01);
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result r1 = cvt.in(state01, e_lit, e_lit + size, efrom_next,
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i_arr, i_arr + size, ito_next);
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VERIFY( r1 == codecvt_base::ok );
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VERIFY( !int_traits::compare(i_arr, i_lit, size) );
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VERIFY( efrom_next == e_lit + size );
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VERIFY( ito_next == i_arr + size );
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</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>
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a. things that are sketchy, or remain unimplemented:
|
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do_encoding, max_length and length member functions
|
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are only weakly implemented. I have no idea how to do
|
||
this correctly, and in a generic manner. Nathan?
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||
</p></li><li><p>
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b. conversions involving std::string
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</p><div class="itemizedlist"><ul type="circle"><li><p>
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how should operators != and == work for string of
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different/same encoding?
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||
</p></li><li><p>
|
||
what is equal? A byte by byte comparison or an
|
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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="id453116"></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="id517137"></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="id517166"></a><p><span class="title"><i>
|
||
ISO/IEC 14882:1998 Programming languages - C++
|
||
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System Interface Definitions, Issue 6 (IEEE Std. 1003.1-200x)
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Addison Wesley
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</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">
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