ISO C++
codecvt

# 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.

## Requirements

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.

## Design

### wchar_t Size

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.

### Support for Unicode

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.

### Other Issues

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.

## Implementation

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.

## Use

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 );

## Future

- 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?

## Bibliography

### The GNU C Library

McGrath
Roland
Drepper
Ulrich
2007
FSF
Chapters 6 Character Set Handling and 7 Locales and Internationalization

### Correspondence

Drepper
Ulrich
2002

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

1998
ISO

### ISO/IEC 9899:1999 Programming languages - C

1999
ISO

### The Open Group Base Specifications, Issue 6 (IEEE Std. 1003.1-2004)

1999
The Open Group/The Institute of Electrical and Electronics Engineers, Inc.

### The C++ Programming Language, Special Edition

Stroustrup
Bjarne
2000
Addison Wesley, Inc.
Appendix D
Addison Wesley

### Standard C++ IOStreams and Locales

Advanced Programmer's Guide and Reference
Langer
Angelika
Kreft
Klaus
2000
Addison Wesley Longman, Inc.
Addison Wesley Longman

### A brief description of Normative Addendum 1

Feather
Clive
Extended Character Sets

### The Unicode HOWTO

Haible
Bruno

### UTF-8 and Unicode FAQ for Unix/Linux

Khun
Markus