Glib::ustring::format is throwing a Glib::ConvertError for some Unicode strings (eg., "Užduotys") which are
being passed to it as char *. There is no problem if the same strings are passed as const char * or Glib::ustring.

The problem arises from the fact that pushing a char * or const char * that points to a Unicode string to a std::wostringstream is resulting in invalid Unicode code points. In this case, the code point corresponding to the letter ž is getting corrupted. These invalid codes are disliked by g_ucs4_to_utf8, which is invoked from Glib::ustring::FormatStream::to_string, and we end up with an exception. (For what it is worth, std::wcout, std::cout are skipping the invalid codepoints instead of blowing up.)

This problem manifests itself only for char * and not const char * because Glib::ustring::FormatStream::stream is overloaded for const char * to create a Glib::ustring from the input parameter, which is in turn pushed into the stream. However there is no such overload to handle char * arguments.