ISO C++ ctype 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. 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. McGrath Roland Drepper Ulrich 2007 FSF Chapters 6 Character Set Handling and 7 Locales and Internationalization Drepper Ulrich 2002 1998 ISO 1999 ISO 1999 The Open Group/The Institute of Electrical and Electronics Engineers, Inc. Stroustrup Bjarne 2000 Addison Wesley, Inc. Appendix D Addison Wesley Advanced Programmer's Guide and Reference Langer Angelika Kreft Klaus 2000 Addison Wesley Longman, Inc. Addison Wesley Longman