gcc/libstdc++-v3/docs/html/22_locale/codecvt.html

<HTML>
<HEAD>
  <H1>
  Notes on the codecvt implementation.
  </H1>
</HEAD>
<I>
prepared by Benjamin Kosnik (bkoz@redhat.com) on August 28, 2000
</I>

<P>
<H2>
1. Abstract
</H2>
<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 &quot;C&quot; (which can now be referred to
as narrow characters.)  This document attempts to describe how the GNU
libstdc++-v3 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>

<P>
<H2>
2. What the standard says
</H2>
Around page 425 of the C++ Standard, this charming heading comes into view:

<BLOCKQUOTE>
22.2.1.5 - Template class codecvt [lib.locale.codecvt]
</BLOCKQUOTE>

The text around the codecvt definition gives some clues:

<BLOCKQUOTE>
<I>
-1- The class codecvt&lt;internT,externT,stateT&gt; 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.
</I>
</BLOCKQUOTE>

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

<BLOCKQUOTE>
<I>
-2- The stateT argument selects the pair of codesets being mapped between.
</I>
</BLOCKQUOTE>

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

<BLOCKQUOTE>
<I>
-3- The instantiations required in the Table ??
(lib.locale.category), namely codecvt&lt;wchar_t,char,mbstate_t&gt; and
codecvt&lt;char,char,mbstate_t&gt;, convert the implementation-defined
native character set. codecvt&lt;char,char,mbstate_t&gt; implements a
degenerate conversion; it does not convert at
all. codecvt&lt;wchar_t,char,mbstate_t&gt; 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.
</I>
</BLOCKQUOTE>

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

<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>

<P>
<H2>
3. Some thoughts on what would be useful
</H2>
Probably the most frequently asked question about code conversion is:
&quot;So dudes, what's the deal with Unicode strings?&quot; 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>
In particular, 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>
Thus, portable C++ code cannot assume a byte size (or endianness) either.

<P>
Getting back to the frequently asked question: What about Unicode strings?

<P>
What magic spell will do this conversion?

<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>

<UL>
<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>

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

<BLOCKQUOTE>
<PRE>
8859_1, 8859_9, 10646-1:1993, 10646-1:1993/UCS4, ARABIC, ARABIC7,
ASCII, EUC-CN, EUC-JP, EUC-KR, EUC-TW, GREEK-CCITT, 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, UNICODELITTLE, US-ASCII, US, UTF-8,
UTF-16, UTF8, UTF16).
</PRE>
</BLOCKQUOTE>

<P>
For iconv-based implementations, string literals for each of the
encodings (ie. &quot;UCS-2&quot; and &quot;UTF-8&quot;) are necessary,
although for other,
non-iconv implementations a table of enumerated values or some other
mechanism may be required.

<LI>
 Maximum length of the identifying string literal.

<LI>
 Some encodings are require explicit endian-ness. As such, some kind
  of endian marker or other byte-order marker will be necessary. See
  &quot;Footnotes for C/C++ developers&quot; in Haible for more information on
  UCS-2/Unicode endian issues. (Summary: big endian seems most likely,
  however implementations, most notably Microsoft, vary.)

<LI>
 Types representing the conversion state, for conversions involving
  the machinery in the &quot;C&quot; 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.

<LI>
 Conversion descriptors for both directions of encoding. (ie, both
  UCS-2 to UTF-8 and UTF-8 to UCS-2.)

<LI>
 Something to indicate if the conversion requested if valid.

<LI>
 Something to represent if the conversion descriptors are valid.

<LI>
 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.
</UL>

<P>
<H2>
4. Problems with &quot;C&quot; code conversions : thread safety, global
locales, termination.
</H2>

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&lt;wchar_t, char, mbstate_t&gt;
when implemented using standard &quot;C&quot; functions.

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

<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>
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>
The last, and fundamental problem, is the assumption of a global
locale for all the &quot;C&quot; 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++-v3 is trying to offer, as an
option, a high-quality implementation, damn the additional complexity!

<P>
For the required specialization codecvt&lt;wchar_t, char, mbstate_t&gt; ,
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>
<H2>
5. Design
</H2>
The two required specializations are implemented as follows:

<P>
<TT>
codecvt&lt;char, char, mbstate_t&gt;
</TT>
<P>
This is a degenerate (ie, does nothing) specialization. Implementing
this was a piece of cake.

<P>
<TT>
codecvt&lt;char, wchar_t, mbstate_t&gt;
</TT>
<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>
Neither of these two required specializations deals with Unicode
characters. As such, libstdc++-v3 implements a partial specialization
of the codecvt class with and iconv wrapper class, __enc_traits as the
third template parameter.

<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>
As such, the type __enc_traits 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>
There are two constructors for __enc_traits:

<P>
<TT>
__enc_traits() : __in_desc(0), __out_desc(0)
</TT>
<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>
<TT>
__enc_traits(const char* __int, const char* __ext)
</TT>
<P>
This constructor takes as parameters string literals that indicate the
desired internal and external encoding. There are no defaults for
either argument.

<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>
<TT>
void
_M_init()
</TT>
<P>
Strangely enough, this member function attempts to open conversion
descriptors for a given __enc_traits 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>
<TT>
bool
_M_good()
</TT>
<P>
Provides a way to see if the given __enc_traits 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>
<TT>
__enc_traits(const __enc_traits&amp;)
</TT>
<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>
Definitions for all the required codecvt member functions are provided
for this specialization, and usage of codecvt&lt;internal character type,
external character type, __enc_traits&gt; is consistent with other
codecvt usage.

<P>
<H2>
6.  Examples
</H2>

<UL>
	<LI>
	a. conversions involving string literals

<pre>
  typedef codecvt_base::result			result;
  typedef unsigned short			unicode_t;
  typedef unicode_t				int_type;
  typedef char					ext_type;
  typedef __enc_traits				enc_type;
  typedef codecvt&lt;int_type, ext_type, enc_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 );
</pre>
	<LI>
	b. conversions involving std::string
	<LI>
	c. conversions involving std::filebuf and std::ostream
</UL>

More information can be found in the following testcases:
<UL>
<LI> testsuite/22_locale/codecvt_char_char.cc
<LI> testsuite/22_locale/codecvt_unicode_wchar_t.cc
<LI> testsuite/22_locale/codecvt_unicode_char.cc
<LI> testsuite/22_locale/codecvt_wchar_t_char.cc
</UL>

<P>
<H2>
7.  Unresolved Issues
</H2>
<UL>
<LI>
	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?

<LI>
	b. conversions involving std::string

	<UL>
		<LI>
		how should operators != and == work for string of
		different/same encoding?

		<LI>
		what is equal? A byte by byte comparison or an
		encoding then byte comparison?

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

		<LI>
		how to synchronize the &quot;C&quot; and &quot;C++&quot;
        conversion information?

		<LI>
		wchar_t/char internal buffers and conversions between
		internal/external buffers?
	</UL>
</UL>

<P>
<H2>
8. Acknowledgments
</H2>
Ulrich Drepper for the iconv suggestions and patient answering of
late-night questions, Jason Merrill for the template partial
specialization hints, language clarification, and wchar_t fixes.

<P>
<H2>
9. Bibliography / Referenced Documents
</H2>

Drepper, Ulrich, GNU libc (glibc) 2.2 manual. In particular, Chapters &quot;6. Character Set Handling&quot; and &quot;7 Locales and Internationalization&quot;

<P>
Drepper, Ulrich, Numerous, late-night email correspondence

<P>
Feather, Clive, &quot;A brief description of Normative Addendum 1,&quot; in particular the parts on Extended Character Sets
http://www.lysator.liu.se/c/na1.html

<P>
Haible, Bruno, &quot;The Unicode HOWTO&quot; v0.18, 4 August 2000
ftp://ftp.ilog.fr/pub/Users/haible/utf8/Unicode-HOWTO.html

<P>
ISO/IEC 14882:1998 Programming languages - C++

<P>
ISO/IEC 9899:1999 Programming languages - C

<P>
Khun, Markus, &quot;UTF-8 and Unicode FAQ for Unix/Linux&quot;
http://www.cl.cam.ac.uk/~mgk25/unicode.html

<P>
Langer, Angelika and Klaus Kreft, Standard C++ IOStreams and Locales, Advanced Programmer's Guide and Reference, Addison Wesley Longman, Inc. 2000

<P>
Stroustrup, Bjarne, Appendix D, The C++ Programming Language, Special Edition, Addison Wesley, Inc. 2000

<P>
System Interface Definitions, Issue 6 (IEEE Std. 1003.1-200x)
The Open Group/The Institute of Electrical and Electronics Engineers, Inc.
http://www.opennc.org/austin/docreg.html