Parameter pack excercise

typelist.cc

#include <cstddef>

using namespace std;

template <size_t N, typename Car, typename Cdr>
struct Nth {
  static_assert(N > 0, "Index out of range.");
  using Type = typename Cdr::template Nth<N - 1>::Type;
};

template <typename Car, typename Cdr>
struct Nth<1, Car, Cdr> { using Type = Car; };

template <typename ...Types>
struct TypeList {
  template <typename Kar, typename ...Kdr>
  struct TypeList1 {
    using Car = Kar;
    using Cdr = TypeList<Kdr...>;
  };

  using Car = typename TypeList1<Types...>::Car;
  using Cdr = typename TypeList1<Types...>::Cdr;

  constexpr static bool IsNull = false;
  constexpr static size_t Size = 1 + Cdr::Size;

  template <size_t N>
  using Nth = Nth<N, Car, Cdr>;
};

template <>
struct TypeList<> {
  using Car = void;
  using Cdr = void;

  constexpr static bool IsNull = true;
  constexpr static size_t Size = 0;

  template <size_t N>
  struct Nth { using Type = void; };
};

#include <iostream>

int main(int, char **) {
  using TypeList = TypeList<int, double>;
  TypeList::Nth<1>::Type v = 42;
  TypeList::Nth<2>::Type u = 3.14;
  cout << TypeList::Size << endl;

  return 0;
}