Hello, in the current implementation system_error::what() returns something like runtime_error::what() + " : " + system_message(); which results in some ugly ' : no such device' format if I only want a system specific error description returned. I suggest that system_error gets two ctors: system_error( error_code const&); // what() returns only system specific error description provided by system_message system_error( error_code const&, std::string const& desc) // what() returns desc which could be formated by the user as needed const char * system_error::what() const throw() { return runtime_error::what(); } regards, Oliver