Select template by range

tmpl_range.cc

#include <iostream>

struct default_tag {};
struct case1_tag {};
struct case2_tag {};

template <size_t N> using selected_t = typename std::conditional<
  10 <= N && N <= 20,
  case1_tag,
  typename std::conditional<
    20 <= N && N <= 30,
    case2_tag,
    default_tag>::type
>::type;

template<size_t N, typename = selected_t<N>> struct X {
 static const int v = 0;
};

template<size_t N> struct X<N, case1_tag> {
 static const int v = 1;
};

template<size_t N> struct X<N, case2_tag> {
 static const int v = 2;
};


int main() {
    std::cout << X<2>::v << '\n';
    std::cout << X<12>::v << '\n';
    std::cout << X<22>::v << '\n';
}