型に連番を割り当てる

sample.cpp

#include <cstddef>
#include <iostream>
#include <type_traits>

// 型に結び付いた連番を生成するクラステンプレート
template <class... T>
class identify_number_traits {
  private:
    template <class U>
    struct foo {
        template <std::size_t m, class V, class... W>
        class bar : public std::integral_constant<std::size_t, bar<m + 1, W...>::value> {};
        template <std::size_t m, class... W>
        class bar<m, U, W...> : public std::integral_constant<std::size_t, m> {};
    };

  public:
    template <class U>
    class type : public std::integral_constant<std::size_t, foo<U>::template bar<0, T...>::value> {}
};

// なんやかんやの適当なクラス
class banana {};
class apple {};
class cherry {};

// 適当なクラスを連番に対応付ける
template <class T>
using fruits_identify = identify_number_traits<banana, apple, cherry>::type<T>;

int main() {
    std::cout << fruits_identify<banana>::value << std::endl
              << fruits_identify<apple>::value << std::endl
              << fruits_identify<cherry>::value << std::endl;
    return 0;
}