It looks like two maps with the same entries but different ordering are considered different. I'm trying to create a set of maps and I'm inserting two maps. Both maps are identical but the order in which the items were inserted into the map is different. I was thinking that an associative sequence like set is using is_same to compare two elements and I tried to overload is_same -- but this did not work. See code below or attached. #include <boost/mpl/set.hpp> #include <boost/mpl/map.hpp> #include <boost/mpl/bool.hpp> #include <boost/mpl/long.hpp> #include <boost/mpl/for_each.hpp> #include <boost/mpl/insert.hpp> #include <iostream> ? struct print { template<typename T> void operator()(const T &_r) const { //std::cout << typeid(T).name() << "\n"; (*this)(_r, typename boost::mpl::is_sequence<T>::type()); } template<typename T> void operator()(const T &, const boost::mpl::true_ &) const { std::cout << "(\n"; boost::mpl::for_each<T>(print()); std::cout << ")\n"; } template<typename T0, typename T1> void operator()(const boost::mpl::pair<T0, T1> &, const boost::mpl::false_ &) const { std::cout << "pair(" << typeid(T0).name() << ", " << typeid(T1).name() << ")" << "\n"; } template<typename T> void operator()(const T &, const boost::mpl::false_ &) const { std::cout << typeid(T).name() << "\n"; } }; namespace boost { //namespace type_traits //{ //template<typename T0, typename T1> //struct is_same; ? using boost::mpl::map; using boost::mpl::true_; using boost::mpl::insert; ? template<typename T0, typename T1> struct is_same<map<T0, T1>, map<T1, T0> > { typedef boost::mpl::true_ type; }; ? template<typename T0, typename T1, typename T2> struct is_same<map<T0, T1, T2>, map<T2, T1, T0> > { typedef boost::mpl::true_ type; }; ? template<typename T0, typename T1, typename T2> struct is_same<map<T0, T1, T2>, map<T1, T2, T0> > { typedef boost::mpl::true_ type; }; ? template<typename T0, typename T1, typename T2> struct is_same<map<T0, T1, T2>, map<T0, T2, T1> > { typedef boost::mpl::true_ type; }; ? template<typename T0, typename T1, typename T2> struct is_same<map<T0, T1, T2>, map<T1, T0, T2> > { typedef boost::mpl::true_ type; }; //} } int main(int argc, char **argv) { using boost::mpl::long_; using boost::mpl::for_each; using boost::mpl::set; using boost::mpl::map; using boost::mpl::pair; using boost::mpl::insert; typedef long_<0> zero; typedef long_<1> one; typedef long_<2> two; typedef pair< zero, one

zero_2_one; typedef pair< one, one one_2_one; for_each< insert< insert< set<>, insert< insert< map<>, zero_2_one ::type, one_2_one ::type ::type, insert< insert< map<>, one_2_one ::type, zero_2_one ::type ::type (print()); return 0; }