I asked earlier in a post "[mpl] Working with map, copy and back_inserter". This is a related question. The common question is: What does compile-time equality means in MPL, particularly for map and set key equality? Does it mean std::is_same? The following code works whether USE_VECTOR_C is defined as 0 or 1 -- the last 2 lines are static_asserts that have conditions determined by USE_VECTOR_C so that they succeed, highlighting the difference in behavior between USE_VECTOR_C == 0 and USE_VECTOR_C == 1. USE_VECTOR_C == 0 is the normal case where everything is working as I expect. USE_VECTOR_C == 1 is the unexpected case where the static_asserts in the last 2 lines have conditions altered so that they succeed. Why do vector_c and vector give me different "element values"? In general, what should I be careful about when trying to use the "compile-time values" as keys to avoid the situation elucidated by the last line, where I expect two "compile-time values" to be the same but they are not, resulting in two set/map items instead of one? To clarify, I'm using boost 1.63 with gcc 6.3.1 and -std=c++14 on Arch Linux. Josh #include <type_traits> #include <boost/mpl/at.hpp> #include <boost/mpl/insert.hpp> #include <boost/mpl/set.hpp> #include <boost/mpl/size.hpp> #include <boost/mpl/size_t.hpp> #include <boost/mpl/vector.hpp> #include <boost/mpl/vector_c.hpp> using namespace std; namespace mpl = boost::mpl; #define USE_VECTOR_C 0 using Zero = mpl::size_t<0>; #if USE_VECTOR_C == 1 using Vector = mpl::vector_c<size_t, 0>; #else using Vector = mpl::vector<Zero>; #endif using ZeroElt = typename mpl::at<Vector, Zero>::type; using EmptySet = mpl::set<>; using NonEmptySet = typename mpl::insert<typename mpl::insert<EmptySet, Zero>::type, ZeroElt>::type; static_assert(ZeroElt::value == 0, ""); static_assert(is_same<ZeroElt::value_type, size_t>::value, ""); static_assert(is_same<ZeroElt, Zero>::value == !USE_VECTOR_C, ""); static_assert(mpl::size<NonEmptySet>::value == USE_VECTOR_C + 1, "");