This just shows how you can specify traits for enumeration values, including types.

enum_value_traits.cpp

/*
This is a rather contrived example where one option
expressed as an enum leads to a second set of options
that's different for each initial option.

What I really have in mind is configuration of a peripheral on microcontroller.
E.g. you first choose serial port 1, and then that gives you some options for
the TX and RX pins which are specific to serial port 1.
*/

// The enum for the primary option
enum class Options { A, B };

enum class OptionASubOptions { A1, A2 };
enum class OptionBSubOptions { B1, B2 };

// The default will just cause compilation failure since the sub_option_type isn't defined
template<Options E>
struct option_traits { };

// Specialise for option A
template<>
struct option_traits<Options::A> {
  typedef OptionASubOptions sub_option_type;
};

// Specialise for option B
template<>
struct option_traits<Options::B> {
  typedef OptionBSubOptions sub_option_type;
};

template<Options Option>
class Foo {
public:
  // The compiler can't work out option_traits<Option>::sub_option_type is
  // a typename, so we need to tell it explicitely that it is
  typedef typename option_traits<Option>::sub_option_type sub_option_type;
  void choose(sub_option_type sub) {
    // Don't do anything - the point is just to show we can compile an example
  }
};

int main(void) {
  // Choose option A:
  Foo<Options::A> foo;
  // Choose sub-option A1
  foo.choose(OptionASubOptions::A2);
  return 0;
}