Runtime index for choosing type

functor-type-switch.cpp

// compile: clang++ -Wall -std=c++11 functor-type-switch.cpp -o functor-type-switch

#include <vector>
#include <iostream>
#include <stdexcept>

template <template<typename> class Func, typename ...Ar>
auto dispatch_type(size_t type, Ar&&... rg) ->
    decltype(Func<int>()(std::forward<Ar>(rg)...)) {
  switch(type) {
    case 0: return Func<int>()(std::forward<Ar>(rg)...);
    case 1: return Func<double>()(std::forward<Ar>(rg)...);
    default: throw std::runtime_error("error");
  }
}

template <class T>
struct Length {
  size_t operator()(size_t len) {
    std::vector<T> vec(len, 100);
    return vec.size();
  }
};

size_t length(size_t type, size_t len) {
  return dispatch_type<Length>(type, len);
}

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;
}