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 #include #include #include #include #include #include #include #include using namespace std; namespace mpl = boost::mpl; using Empty = typename mpl::vector<>; using Count = mpl::vector, mpl::size_t<1>>; using ConstructedCount = typename mpl::copy< typename mpl::range_c, mpl::back_inserter<Empty>>::type; using The0 = typename mpl::at>::type; using The1 = typename mpl::at>::type; static_assert(mpl::equal>::value, "succeeds"); static_assert(The1::value == 1, "succeeds"); static_assert(std::is_same::value, "succeeds"); static_assert(std::is_same::value, "fails"); static_assert(std::is_same>::value, "fails");