That makes sense to me. It wouldn't be useful to throw an exception because of the asynchronous nature of the control flow, so supplying a error_code to the callback gives it a chance to deal with the error, or ignore it if desired. Was that your analysis?
Is it necessary to differentiate between error codes and exceptions? An exception *is* an error code for me. IOW, it might make sense to derive an error_code class from std::exception. Assume we have two similar functions f1() and f2() with the same possible failure modes. We'd like f1() to throw and f2() not. The prototypes might look like: void f1(arguments...) throw(std::exception); std::auto_ptr<std::exception> f2(arguments...) throw() ; Regards -Gerhard -- Gerhard Wesp ZRH office voice: +41 (0)44 668 1878 ZRH office fax: +41 (0)44 200 1818 For the rest I claim that raw pointers must be abolished.