num_to_type.cpp

template <bool cond, typename _, typename else_>
struct enable_if { typedef typename else_::type type; };
template <typename then_, typename _>
struct enable_if<true, then_, _>{ typedef typename then_::type type; };

template <int n>
struct num {
  static const int value = n;
  typedef num<n> type;
};

template <int n, int byte>
struct num_to_byte_impl {
  static const bool cond = n > (2 << (8 * byte - 1)) - 1;
  typedef typename enable_if<cond,
                             num_to_byte_impl<n, byte+1>,
                             num<byte> >::type type;
};

template <int n>
struct num_to_byte {
  typedef typename num_to_byte_impl<n, 1>::type type;
  static const int value = type::value;
};

template <int n>
struct byte_to_type;
template <> struct byte_to_type<1> { typedef char type; };
template <> struct byte_to_type<2> { typedef short type; };
template <> struct byte_to_type<4> { typedef int type; };
template <> struct byte_to_type<8> { typedef long long type; };

template <int n>
struct num_to_type {
  static const int byte = num_to_byte<n>::value;
  typedef typename byte_to_type<byte>::type;
};