Templates - predefined configurations

Use case:
  There is a fixed number of configurations for which we want to define a given parameter value.

template<int Category, typename T=void> struct ConfigurationValue {
  static_assert(false, "Not defined");
};
template<> struct ConfigurationValue<2> { enum { value = 2 }; };
template<> struct ConfigurationValue<3> { enum { value = 7 }; };