Runtime index for choosing type

mp11-type-list-switch.cpp

// compile: clang++ -Wall -std=c++14 -I /usr/local/include mp11-type-list-switch.cpp -o mp11-type-list-switch

#include <vector>
#include <iostream>

#include <boost/mp11/list.hpp>
#include <boost/mp11/algorithm.hpp>

namespace mp11 = boost::mp11;

using num_types = mp11::mp_list<int, double>;
template <size_t i>
using num_type = mp11::mp_at_c<num_types, i>

template<class F>
inline constexpr
    decltype(std::declval<F>()(std::declval<mp11::mp_size_t<0>>()))
    with_type(std::size_t i, F && f) {

  return mp11::mp_with_index< mp11::mp_size<num_types> >(i,
      std::forward<F>(f));
}

size_t length(size_t i, size_t len) {
  return with_type(i, [&](auto I) {
    std::vector< num_type<I> > vec(len, 100);
    return vec.size();
  });
}

int main () {

  std::cout << "length: " << length(0, 4) << "\n";
  std::cout << "length: " << length(1, 5) << "\n";
  std::cout << "length: " << length(2, 6) << "\n";

  return 0;
}