I have a callback-interface that can optionally process a "ServerError". If the callback looks at the error, there should be no default processing of the error. I have created a class callback that accepts two kinds of Boost.Functions: one with, one without an error object. When passing a callback function that does not look at the error, the call is ambiguous: both Boost.Functions could accept it. Is there a way to work around that problem? Can callback detect what kind of function it was passed? Some code illustrating my question: #include <boost/function.hpp> class ServerError; template<typename T> struct callback { typedef boost::function<void(T)> cb_func; cb_func callback_; typedef boost::function<void(T,const ServerError&)> cb_err_func; cb_err_func callback_err_; callback(const cb_func &cb) : callback_(cb) {} callback(const cb_err_func &cb) : callback_err_(cb) {} }; static void xxx(int) {} int main (int argc, char **argv) { // does not compile: ambiguous... callback<int> cb(&xxx); // does compile: but I'd like the compiler (or my lib) to automatically pick // the right one callback<int> cb(callback<int>::cb_func(&xxx)); return 0; } In the meantime I have found a possible workaround: If I change typedef boost::function<void(T,const ServerError&)> cb_err_func; to typedef boost::function<void(const ServerError&,T)> cb_err_func; the code does compile. Best regards and thank you for looking into this Christoph