Can enum class be nested?

Question

Can this be done?

enum A
{
    enum B
    {
        SOMETHING1,
        SOMETHING2
    };

    enum C
    {
        SOMETHING3,
        SOMETHING4
    };
};
         


        

Answer 1

MESONS pi = PI();
MATTER mat = pi;
assert (pi == mat);

Do you mind a little C++11 template magic?

template 
struct is_defined_in : std::false_type {};

template 
struct is_defined_in : is_defined_in {};

template 
struct is_defined_in : std::true_type {};

template  struct helper {
  friend bool operator==(helper a, helper b)
  { return a.id == b.id; }
  friend bool operator!=(helper a, helper b)
  { return a.id != b.id; }

  int id=ID;
};
template  struct category {
  int id;

  template ::value>::type>
  category(T t) : id(t.id) {}

  friend bool operator==(category a, category b)
  { return a.id == b.id; }
  friend bool operator!=(category a, category b)
  { return a.id != b.id; }
};

enum class ElementaryParticleTypesID
{ PI, PROTON, NEUTRON, ELECTRON };

struct PI       : helper<(int)ElementaryParticleTypesID::PI> {};
struct PROTON   : helper<(int)ElementaryParticleTypesID::PROTON> {};
struct NEUTRON  : helper<(int)ElementaryParticleTypesID::NEUTRON> {};
struct ELECTRON : helper<(int)ElementaryParticleTypesID::ELECTRON> {};

using MESONS = category;
using BARYONS = category;
using LEPTONS = category;

using MATTER = category;

(the static_assert currently doesn't work beyond two hierarchies, but this can be added if you want to)