Hello, I think there are some bugs in the implementation: void system_message( error_code ec, std::string & target ) { target += std::strerror( sys_err_code ); // shoudn't this be target += std::strerror( ec); } In the documentation the signature of the function should be std::string& system_message(error_code err, std::string& target); std::string& system_message( error_code ec, std::string & target ) { target += std::strerror( ec); return target; } regards, Oliver