Example implementation stripping the last few values of a tuple, expanded in conditional tuple example

conditional_tuple_type.cc

// Author:          Julius Hülsmann
// Email:           juliusHuelsmann [at] gmail.com, julius.huelsmann [at] data-spree.com
// Creation Date:   2020-02-18
// Execution: g++-10.0 -std=c++2a conditional_tuuple_type.cc


// Example implementation stripping the last few values of a tuple, expanded in conditional tuple example

#include <tuple>

template <size_t s, typename is, typename ...args> struct thlp;

template <size_t s, size_t... i, class... args> struct thlp<s, std::index_sequence<i...>, args...> {
  static_assert(s <= sizeof...(args), "Requested new tuple size exceeds old one.");
  using type = std::tuple<std::tuple_element_t<i, std::tuple<args...>>...>;
};

template <size_t s, typename... args>
using conditional_tuple_t = thlp<s, decltype(std::make_index_sequence<s>()), args...>::type;

int main() {
  struct exmpl { exmpl(){} virtual ~exmpl(){} }; //< example struct with virtual destructor
  static_assert(std::is_same_v<std::tuple<exmpl, int>, conditional_tuple_t<2, exmpl, int, double>>);
}