Runtime index for choosing type

mp11-functor-recursion.cpp

// compile: clang++ -Wall -std=c++11 -I /usr/local/include mp11.cpp -o mp11

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

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

namespace mp11 = boost::mp11;

using num_types = mp11::mp_list<int, long, float, double>;

template <typename T, std::size_t N> struct dispatch_impl {
  template <std::size_t K, template<typename> class Func, typename ...Ar>
  static auto call(size_t i, Ar&&... rg) ->
      decltype(Func< mp11::mp_at_c<T, 0> >()(std::forward<Ar>(rg)...)) {
    if (i == 0) {
      std::cout << "hello" << "\n";
      return Func< mp11::mp_at_c<T, K> >()(std::forward<Ar>(rg)...);
    } else {
      return dispatch_impl<T, N - 1>::template call<K + 1, Func>(i - 1,
          std::forward<Ar>(rg)...);
    }
  }
};

template <typename T> struct dispatch_impl<T, 0> { };

template <typename T> struct dispatch_impl<T, 1> {
  template <std::size_t K, template<typename> class Func, typename ...Ar>
  static auto call(size_t i, Ar&&... rg) ->
      decltype(Func< mp11::mp_at_c<T, 0> >()(std::forward<Ar>(rg)...)) {
    if (i == 0) {
      return Func< mp11::mp_at_c<T, K> >()(std::forward<Ar>(rg)...);
    } else {
      throw std::runtime_error("error");
    }
  }
};

template <template<typename> class Func, typename ...Ar>
auto dispatch_type(size_t type, Ar&&... rg) ->
    decltype(Func< mp11::mp_at_c<num_types, 0> >()(std::forward<Ar>(rg)...)) {
  using N = mp11::mp_size<num_types>;
  return dispatch_impl< num_types, std::size_t{N::value} >::template call<0,
      Func>(type, std::forward<Ar>(rg)...);
}

template <class T>
struct Length {
  size_t operator()(size_t len) {
    std::vector<T> vec(len, 100);
    return vec.size();
  }
};
template <>
struct Length <int> {
  size_t operator()(size_t len) {
    std::vector<int> vec(len + 1, 100);
    return vec.size();
  }
};
template <>
struct Length <float> {
  size_t operator()(size_t len) {
    std::vector<float> vec(len + 2, 100);
    return vec.size();
  }
};
template <>
struct Length <double> {
  size_t operator()(size_t len) {
    std::vector<double> vec(len + 3, 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, 0) << "\n";
  std::cout << "length: " << length(1, 0) << "\n";
  std::cout << "length: " << length(2, 0) << "\n";
  std::cout << "length: " << length(3, 0) << "\n";
  std::cout << "length: " << length(4, 0) << "\n";
  std::cout << "length: " << length(5, 0) << "\n";

  return 0;
}