c++ - Convert scalar + std::array + std::tuple into a big tuple -

consider following code:

template <class scalar, class array, class tuple> class test {}; 

where array std::array, , tuple std::tuple. in class, have lot of sfinae, , create big tuple called types contain complete list of types. allow me test conditions variadic lists.

so challenge create type have following behaviour. if:

• scalar = int
• array = std::array<double, 3>
• tuple = std::tuple<char, float, std::string>

then:

• types = std::tuple<int, double, double, double, char, float, std::string>

which concatenation of internal data of scalar, array , tuple.

how ?

this seems work:

template<typename t1, typename t2> struct concat_tuples;  template<typename... t1, typename... t2> struct concat_tuples<std::tuple<t1...>, std::tuple<t2...>> {     using type = std::tuple<t1..., t2...>; };   // n_tuple<int, 3>::type == std::tuple<int, int, int> template<typename t, size_t n> struct n_tuple;  template<typename t> struct n_tuple<t, 0>  {     using type = std::tuple<>; };  template<typename t, size_t n> struct n_tuple {     using type = typename concat_tuples<                     typename n_tuple<t, n-1>::type,                      std::tuple<t>                 >::type; };  template <class scalar, class array, class tuple> struct test;  template <class scalar,  typename t, size_t n, typename... ts> struct test<scalar, std::array<t, n>, std::tuple<ts...>> {     using type = typename concat_tuples<                     typename concat_tuples<                         std::tuple<scalar>,                          typename n_tuple<t, n>::type                     >::type,                      std::tuple<ts...>                 >::type; }; 

live demo here.