Hi All, I am using the boost::variant with the binary visitor algorithm, apply_visitor. struct UnknownValue {}; typedef boost::variant<UnknownValue, long, unsigned long, double, string, char> ValueType; I am using apply_visitor for binary visitation as below: class AddVisitor : public boost::static_visitor<> { public: template <typename T> void operator()(const T &lhs, const T &rhs) { T val = lhs + rhs; } template <typename T, typename R> void operator()(const T &lhs, const R &rhs) { // do nothing...different types } template <typename T> void operator()(const T &lhs, const UnknownValue &rhs) { // do nothing...we have an unknown value } template <typename T> void operator()(const UnknownValue &lhs, const T &rhs) { // do nothing...we have an unknown value } void operator()(const UnknownValue &lhs, const UnknownValue &rhs) { // do nothing...we have an unknown value } }; The above is not complete because I can do addition if the types are long and unsigned long or long and double or long and char...etc. What is the preferred method for doing this? Do I need to create an overridden operator() for every argument combination that is valid or can I get the argument types in the function? Thanks Glenn