libstdc++-v3/doc/xml/manual/codecvt.xml

<section xmlns="http://docbook.org/ns/docbook" version="5.0"
	 xml:id="std.localization.facet.codecvt" xreflabel="codecvt">
<?dbhtml filename="codecvt.html"?>

<info><title>codecvt</title>
  <keywordset>
    <keyword>ISO C++</keyword>
    <keyword>codecvt</keyword>
  </keywordset>
</info>



<para>
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 <type>wchar_t</type>) and the standard
type <type>char</type> that is so beloved in classic <quote>C</quote>
(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.
</para>

<section xml:id="facet.codecvt.req"><info><title>Requirements</title></info>


<para>
Around page 425 of the C++ Standard, this charming heading comes into view:
</para>

<blockquote>
<para>
22.2.1.5 - Template class codecvt
</para>
</blockquote>

<para>
The text around the codecvt definition gives some clues:
</para>

<blockquote>
<para>
<emphasis>
-1- The class <code>codecvt&lt;internT,externT,stateT&gt;</code> 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.
</emphasis>
</para>
</blockquote>

<para>
Hmm. So, in some unspecified way, Unicode encodings and
translations between other character sets should be handled by this
class.
</para>

<blockquote>
<para>
<emphasis>
-2- The <type>stateT</type> argument selects the pair of codesets being mapped between.
</emphasis>
</para>
</blockquote>

<para>
Ah ha! Another clue...
</para>

<blockquote>
<para>
<emphasis>
-3- The instantiations required in the Table 51 (lib.locale.category), namely
<classname>codecvt&lt;wchar_t,char,mbstate_t&gt;</classname> and
<classname>codecvt&lt;char,char,mbstate_t&gt;</classname>, convert the
implementation-defined native character set.
<classname>codecvt&lt;char,char,mbstate_t&gt;</classname> implements a
degenerate conversion; it does not convert at all.
<classname>codecvt&lt;wchar_t,char,mbstate_t&gt;</classname> converts between
the native character sets for tiny and wide characters. Instantiations on
<type>mbstate_t</type> perform conversion between encodings known to the library
implementor.  Other encodings can be converted by specializing on a
user-defined <type>stateT</type> type. The <type>stateT</type> object can
contain any state that is useful to communicate to or from the specialized
<function>do_convert</function> member.
</emphasis>
</para>
</blockquote>

<para>
At this point, a couple points become clear:
</para>

<para>
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, <type>stateT</type>.</para>

<para>
Two: The required conversions, by specifying <type>mbstate_t</type> as the
third template parameter, imply an implementation strategy that is mostly
(or wholly) based on the underlying C library, and the functions
<function>mcsrtombs</function> and <function>wcsrtombs</function> in
particular.</para>
</section>

<section xml:id="facet.codecvt.design"><info><title>Design</title></info>


<section xml:id="codecvt.design.wchar_t_size"><info><title><type>wchar_t</type> Size</title></info>


    <para>
      The simple implementation detail of <type>wchar_t</type>'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 <type>wchar_t</type>.
    </para>

    <para>
      Thus, portable C++ code cannot assume a byte size (or endianness) either.
    </para>
  </section>

<section xml:id="codecvt.design.unicode"><info><title>Support for Unicode</title></info>

  <para>
    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. The Unicode character
    set (and useful encodings like UTF-8, UCS-4, ISO 8859-10,
    etc etc etc) were not mentioned in the first C++ standard. (The 2011
    standard added support for string literals with different encodings
    and some library facilities for converting between encodings, but the
    notes below have not been updated to reflect that.)
  </para>

  <para>
    A couple of comments:
  </para>

  <para>
    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 <type>stateT</type> 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:
  </para>

  <itemizedlist>
    <listitem>
      <para>
	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:
      </para>

      <para>
	(An edited list taken from <code>`iconv --list`</code> on a
	Red Hat 6.2/Intel system:
      </para>

<blockquote>
<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).
</programlisting>
</blockquote>

<para>
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.
</para>
</listitem>

<listitem><para>
 Maximum length of the identifying string literal.
</para></listitem>

<listitem><para>
 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.)
</para></listitem>

<listitem><para>
 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.
</para></listitem>

<listitem><para>
 Conversion descriptors for both directions of encoding. (i.e., both
  UCS-2 to UTF-8 and UTF-8 to UCS-2.)
</para></listitem>

<listitem><para>
 Something to indicate if the conversion requested if valid.
</para></listitem>

<listitem><para>
 Something to represent if the conversion descriptors are valid.
</para></listitem>

<listitem><para>
 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.
</para></listitem>
</itemizedlist>
</section>

<section xml:id="codecvt.design.issues"><info><title>Other Issues</title></info>

<para>
In addition, multi-threaded and multi-locale environments also impact
the design and requirements for code conversions. In particular, they
affect the required specialization
<classname>codecvt&lt;wchar_t, char, mbstate_t&gt;</classname>
when implemented using standard "C" functions.
</para>

<para>
Three problems arise, one big, one of medium importance, and one small.
</para>

<para>
First, the small: <function>mcsrtombs</function> and
<function>wcsrtombs</function> may not be multithread-safe
on all systems required by the GNU tools. For GNU/Linux and glibc,
this is not an issue.
</para>

<para>
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!
</para>

<para>
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!
</para>

<para>
For the required specialization
<classname>codecvt&lt;wchar_t, char, mbstate_t&gt;</classname>,
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.
</para>

</section>

</section>

<section xml:id="facet.codecvt.impl"><info><title>Implementation</title></info>


<para>
The two required specializations are implemented as follows:
</para>

<para>
<code>
codecvt&lt;char, char, mbstate_t&gt;
</code>
</para>
<para>
This is a degenerate (i.e., does nothing) specialization. Implementing
this was a piece of cake.
</para>

<para>
<code>
codecvt&lt;char, wchar_t, mbstate_t&gt;
</code>
</para>

<para>
This specialization, by specifying all the template parameters, pretty
much ties the hands of implementors. As such, the implementation is
straightforward, involving <function>mcsrtombs</function> for the conversions
between <type>char</type> to <type>wchar_t</type> and
<function>wcsrtombs</function> for conversions between <type>wchar_t</type>
and <type>char</type>.
</para>

<para>
Neither of these two required specializations deals with Unicode
characters. As such, libstdc++ implements a partial specialization
of the <type>codecvt</type> class with an iconv wrapper class,
<classname>encoding_state</classname> as the third template parameter.
</para>

<para>
This implementation should be standards conformant. First of all, the
standard explicitly points out that instantiations on the third
template parameter, <type>stateT</type>, 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 <type>stateT</type> type elsewhere in the
standard (see 21.1.2 traits typedefs) only indicate that this type be copy
constructible.
</para>

<para>
As such, the type <type>encoding_state</type> is defined as a non-templatized,
POD type to be used as the third type of a <type>codecvt</type> instantiation.
This type is just a wrapper class for iconv, and provides an easy interface
to iconv functionality.
</para>

<para>
There are two constructors for <type>encoding_state</type>:
</para>

<para>
<code>
encoding_state() : __in_desc(0), __out_desc(0)
</code>
</para>
<para>
This default constructor sets the internal encoding to some default
(currently UCS4) and the external encoding to whatever is returned by
<code>nl_langinfo(CODESET)</code>.
</para>

<para>
<code>
encoding_state(const char* __int, const char* __ext)
</code>
</para>

<para>
This constructor takes as parameters string literals that indicate the
desired internal and external encoding. There are no defaults for
either argument.
</para>

<para>
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.
</para>

<para>
<code>
void
_M_init()
</code>
</para>
<para>
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.
</para>

<para>
<code>
bool
_M_good()
</code>
</para>

<para>
Provides a way to see if the given <type>encoding_state</type> 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 <function>iconv_open</function> could not allocate
conversion descriptors, this will also return false. Otherwise, the object is
ready to convert and will return true.
</para>

<para>
<code>
encoding_state(const encoding_state&amp;)
</code>
</para>

<para>
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.
</para>

<para>
Definitions for all the required codecvt member functions are provided
for this specialization, and usage of <code>codecvt&lt;<replaceable>internal
character type</replaceable>, <replaceable>external character type</replaceable>, <replaceable>encoding_state</replaceable>&gt;</code> is consistent with other
codecvt usage.
</para>

</section>

<section xml:id="facet.codecvt.use"><info><title>Use</title></info>

<para>A conversion involving a string literal.</para>

<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&lt;int_type, ext_type, state_type&gt; 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&lt;unicode_codecvt&gt;(loc) );
  const unicode_codecvt&amp; cvt = use_facet&lt;unicode_codecvt&gt;(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 );
</programlisting>

</section>

<section xml:id="facet.codecvt.future"><info><title>Future</title></info>

<itemizedlist>
<listitem>
  <para>
   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?
</para>
</listitem>

<listitem>
  <para>
   b. conversions involving <type>std::string</type>
  </para>
   <itemizedlist>
      <listitem><para>
      how should operators != and == work for string of
      different/same encoding?
      </para></listitem>

      <listitem><para>
      what is equal? A byte by byte comparison or an
      encoding then byte comparison?
      </para></listitem>

      <listitem><para>
      conversions between narrow, wide, and unicode strings
      </para></listitem>
   </itemizedlist>
</listitem>
<listitem><para>
   c. conversions involving std::filebuf and std::ostream
</para>
   <itemizedlist>
      <listitem><para>
      how to initialize the state object in a
      standards-conformant manner?
      </para></listitem>

		<listitem><para>
      how to synchronize the "C" and "C++"
      conversion information?
      </para></listitem>

		<listitem><para>
      wchar_t/char internal buffers and conversions between
      internal/external buffers?
      </para></listitem>
   </itemizedlist>
</listitem>
</itemizedlist>
</section>


<bibliography xml:id="facet.codecvt.biblio"><info><title>Bibliography</title></info>


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    </citetitle>
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      <holder>FSF</holder>
    </copyright>
    <pagenums>
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    </pagenums>
  </biblioentry>

  <biblioentry>
    <citetitle>
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    </citetitle>
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	</link>
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    <author><personname><surname>Feather</surname><firstname>Clive</firstname></personname></author>
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</bibliography>

</section>