Runtime index for choosing type

functor-recursive-switch.cpp

#include <stdexcept>
#include <tuple>
#include <iostream>

// terminating case to avoid if-constexpr
template <template<class> class F, typename R, typename... Ar>
R dispatch_impl(int, Ar&&... rgs) {
  throw std::runtime_error("error");
}

// main recursive case
template <template<class> class F, typename R, typename... Ar,
          typename Ty, typename... pes>
R dispatch_impl(int index, Ar&&... rgs) {
  if (index == 0) {
    return F<Ty>::f(std::forward<Ar>(rgs)...);
  } else {
    return dispatch_impl<F, R, Ar..., pes...>(index - 1,
        std::forward<Ar>(rgs)...);
  }
}

template <template<class> class F, typename... Ar>
auto dispatch_type(int index, Ar&&... rgs) {
  return dispatch_impl<F, int, Ar..., int, char>(index,
      std::forward<Ar>(rgs)...);
}

template <typename T> struct foo;

template <> struct foo<int> { static int f(int i) {return 1;} };
template <> struct foo<char> { static int f(int i) {return 2;} };

int main(void) {
  std::cout << "int: " << dispatch_type<foo>(1, 1) << "\n";
}