It seems very simple, but it's not compiling. int translate(const std::string& str) { return 0; } //all required using declarations are present but omitted for brevity // options_description options; variables_map values; int i; options.add_options() ("test", valuestd::string()->notifier(var(i) = translate(_1))); I get the following error: 'translate': cannot convert parameter 1 from 'boost::lambda::placeholder1_type' to 'std::string'. No user defined conversion operator available that can perform this conversion, or the operator cannot be called. Is this type of usage unsupported for some reason? If I change the variable declaration and the add_options() call to the following it works fine: std::string s options.add_options() ("test", valuestd::string()->notifier(var(s) = _1));