I'm trying to implement a compile-time topological sort of the DAG representing a certain dependency relationship between my classes. One of the data structures I can use to implement topological sort is a map, so I'm trying to use mpl::map using mpl::size_t<> as key. The documentation doesn't define key equality for mpl::map, but I assume it's effectively std::is_same<>. If so, I think I have a problem. In the following code, the last two lines fail static_assert(). The essence of the problem is that my attempt to construct/build my keys using copy with back_inserter does not give me a mpl::vector<> of mpl::size_t<> that is_same to Count, the expected value. It does give me a sequence, and the elements have the right value and value_type but they are not is_same to the expected size_t<> types. So, if I use the elements together with the size_t<> in an mpl::map, won't they be considered to be different keys? Josh #include <type_traits> #include <boost/mpl/at.hpp> #include <boost/mpl/back_inserter.hpp> #include <boost/mpl/copy.hpp> #include <boost/mpl/equal.hpp> #include <boost/mpl/equal_to.hpp> #include <boost/mpl/range_c.hpp> #include <boost/mpl/size_t.hpp> #include <boost/mpl/vector.hpp> using namespace std; namespace mpl = boost::mpl; using Empty = typename mpl::vector<>; using Count = mpl::vector<mpl::size_t<0>, mpl::size_t<1>>; using ConstructedCount = typename mpl::copy< typename mpl::range_c<size_t, 0, 2>, mpl::back_inserter<Empty>>::type; using The0 = typename mpl::at<ConstructedCount, mpl::size_t<0>>::type; using The1 = typename mpl::at<ConstructedCount, mpl::size_t<1>>::type; static_assert(mpl::equal<Count, ConstructedCount, mpl::equal_to<mpl::_1, mpl::_2>>::value, "succeeds"); static_assert(The1::value == 1, "succeeds"); static_assert(std::is_same<typename The1::value_type, size_t>::value, "succeeds"); static_assert(std::is_same<Count, ConstructedCount>::value, "fails"); static_assert(std::is_same<The1, mpl::size_t<1>>::value, "fails");