Keywords: ISO C++ , library

Chapter 15. Facets aka Categories
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## Chapter 15. Facets aka Categories

## ctype

### Implementation

#### Specializations

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:

`ctype<char>`

This is simple specialization. Implementing this was a piece of cake.

`ctype<wchar_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.

### Future

- How to deal with the global locale issue?

- How to deal with different types than char, wchar_t?

- Overlap between codecvt/ctype: narrow/widen

- Mask typedef in codecvt_base, argument types in codecvt. what is know about this type?

- Why mask* argument in codecvt?

- Can this be made (more) generic? is there a simple way to straighten out the configure-time mess that is a by-product of this class?

- Get the ctype<wchar_t>::mask stuff under control. Need to make some kind of static table, and not do lookup every time somebody hits the do_is... functions. Too bad we can't just redefine mask for ctype<wchar_t>

- Rename abstract base class. See if just smash-overriding is a better approach. Clarify, add sanity to naming.

### Bibliography

The GNU C Library . Roland McGrath. Ulrich Drepper. Copyright © 2007 FSF. Chapters 6 Character Set Handling and 7 Locales and Internationalization.

Correspondence . Ulrich Drepper. Copyright © 2002 .

ISO/IEC 14882:1998 Programming languages - C++ . Copyright © 1998 ISO.

ISO/IEC 9899:1999 Programming languages - C . Copyright © 1999 ISO.

The Open Group Base Specifications, Issue 6 (IEEE Std. 1003.1-2004) . Copyright © 1999 The Open Group/The Institute of Electrical and Electronics Engineers, Inc.. .

The C++ Programming Language, Special Edition . Bjarne Stroustrup. Copyright © 2000 Addison Wesley, Inc.. Appendix D. Addison Wesley .

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

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