Hi, i came across this issue when trying to use iostreams with a custom character class and custom character traits. The main problem is, that iostreams assumes std::streampos as the position type for all devices. Some parts of iostreams, like detail::indirect_streambuf honor the char traits' custom pos_type, but this will cause build failures as the underlying device (even code_converter) uses std::streampos as position type. My proposal is to add an associated character traits type to a Device. A Device type whould therefore not only provide information about the underlying character type, but also about the used pos_type, off_type and code conversion (not) done. I would even use traits different from std::char_traits<Ch>/iostreams::char_traits<Ch> as default character traits for most devices. Most devices like for example file_descriptor_sink don't do any code conversion, so a pos_type which contains a mbstate_t is useless and leaves the user with the impression, that code conversion *is* done. A simple integral type like C's off_t should do fine. Let's call those traits binary_char_traits for now. As an implication of this a iostreams::stream_buffer<file_descriptor_sink> should use the Device's char traits type, so should be a std::basic_streambuf<char, binary_char_traits<char> >. Especially it whould not be useable with std::iostream. This is IMO a nice and very clean result, as an iostream expects characters on which code conversion has been performed. You whould need to plug a code_converter in between. Some thoughts about iostreams::code_converter. Perhaps we should change the Codecvt template parameter to a second traits type. This traits type will become the code_converter's associated char traits type. the codecvt type could be an extra template parameter or is simply std::codecvt<code_converter_char_type, underlying_device_char_type, codecvt, code_converter_traits_state_type>. this whould be the same mechanism as code conversion in the stdlib works (using std::codecvt specialization). Comments to my thoughts are very welcome. --- Thomas