Say I have a wide-character, user-defined, streamable type like boost::wssub_match from the regex library (or now std::wssub_match). It's basically a pair of wstring iterators. It's stream insertion operator is defined like this: template <class charT, class ST, class BiIter> basic_ostream<charT, ST>& operator<<(basic_ostream<charT, ST>& os, const sub_match<BiIter>& m) { return (os << m.str()); } If it's not obvious, it requires that the stream's character type is the same as the BiIter's value_type. If I were to try to use boost::lexical_cast to cast this to something, it won't work. That's because lexical_cast doesn't know about wssub_match, and so doesn't know that its character type should be wchar_t. Instead, it defaults to char for any unknown types, and doesn't give a way to override this. There is an internal trait, boost::detail::stream_char, that lexical_cast uses to determine what the character type of the source is (if any). It's specialized on only a fixed number of types. Since it's an implementation detail, it can't be used to extend the set of types. Also, I tried specializing it. In past versions of Boost that just worked, but it doesn't anymore. I think being able to extend lexical_cast to support types like (std|boost)::wssub_match is an essential feature. Thoughts on how to get there from here? -- Eric Niebler BoostPro Computing http://www.boostpro.com