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      ISO C++

      codecvt

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 C (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.

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

22.2.1.5 - Template class codecvt

The text around the codecvt definition gives some clues:

-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.

Hmm. So, in some unspecified way, Unicode encodings and

translations between other character sets should be handled by this

class.

-2- The stateT argument selects the pair of codesets being mapped between.

Ah ha! Another clue...

-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.

At this point, a couple points become clear:

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.

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.

      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.

      Thus, portable C++ code cannot assume a byte size (or endianness) either.

    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.

    A couple of comments:

    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:

        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:

        (An edited list taken from `iconv --list` on a

        Red Hat 6.2/Intel system:

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).

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.

 Maximum length of the identifying string literal.

 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.)

 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.

 Conversion descriptors for both directions of encoding. (i.e., both

  UCS-2 to UTF-8 and UTF-8 to UCS-2.)

 Something to indicate if the conversion requested if valid.

 Something to represent if the conversion descriptors are valid.

 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.

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.

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

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.

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!

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!

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.

The two required specializations are implemented as follows:

codecvt<char, char, mbstate_t>

This is a degenerate (i.e., does nothing) specialization. Implementing

this was a piece of cake.

codecvt<char, wchar_t, mbstate_t>

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.

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.

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.

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.

There are two constructors for encoding_state:

encoding_state() : __in_desc(0), __out_desc(0)

This default constructor sets the internal encoding to some default

(currently UCS4) and the external encoding to whatever is returned by

nl_langinfo(CODESET).

encoding_state(const char* __int, const char* __ext)

This constructor takes as parameters string literals that indicate the

desired internal and external encoding. There are no defaults for

either argument.

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.

void

_M_init()

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.

bool

_M_good()

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.

encoding_state(const encoding_state&)

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.

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.

A conversions involving string literal.

  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 );

   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?

   b. conversions involving std::string

      how should operators != and == work for string of

      different/same encoding?

      what is equal? A byte by byte comparison or an

      encoding then byte comparison?

      conversions between narrow, wide, and unicode strings

   c. conversions involving std::filebuf and std::ostream

      how to initialize the state object in a

      standards-conformant manner?

      how to synchronize the "C" and "C++"

      conversion information?

      wchar_t/char internal buffers and conversions between

      internal/external buffers?

      The GNU C Library

    McGrathRoland

    DrepperUlrich

      2007

      FSF

      Chapters 6 Character Set Handling and 7 Locales and Internationalization

      Correspondence

    DrepperUlrich

      2002

      ISO/IEC 14882:1998 Programming languages - C++

      1998

      ISO

      ISO/IEC 9899:1999 Programming languages - C

      1999

      ISO

      2008

        The Open Group/The Institute of Electrical and Electronics

        Engineers, Inc.

      The C++ Programming Language, Special Edition

    StroustrupBjarne

      2000

      Addison Wesley, Inc.

    Appendix D

        Addison Wesley

      Standard C++ IOStreams and Locales

      Advanced Programmer's Guide and Reference

    LangerAngelika

    KreftKlaus

      2000

      Addison Wesley Longman, Inc.

        Addison Wesley Longman

    FeatherClive

    Extended Character Sets

    HaibleBruno

    KhunMarkus