struct declaration along with C++ class public methods?

Question

I'm trying to understand how to access data in this author's class. I don't seem to be able to access via:

auto c = MyClass();
c.SomeData.a;

class MyClass {
 public:

  struct SomeData {
    typedef MyClass SomeType;

    static constexpr uint32_t a;
    ...
  }

  struct SomeOtherData {
    typedef MyClass SomeType;

    static constexpr uint32_t b;
    ...
  }

Are all those anonymous structs different ways to view the same data like a union?

Answer 1

The SomeData and SomeOtherData structs are inner types of MyClass , they are not non-static data fields of MyClass , which is why you can't access them via your c variable the way you are. MyClass would need to declare actual member fields using those inner types, eg:

class MyClass {
public:

  struct SomeData {
    typedef MyClass SomeType;

    static constexpr uint32_t a;
    ...
  };
  SomeData sd;

  struct SomeOtherData {
    typedef MyClass SomeType;

    static constexpr uint32_t b;
    ...
  };
  SomeOtherData sod;

  ...
};

MyClass c;
c.sd.a;
c.sod.a;

But, since the fields you are actually trying to access are static , just refer to them by their owning type only, you don't need a MyClass variable at all, eg:

MyClass::SomeData::a;
MyClass::SomeOtherData::b;

And no, these inner structs are nothing like union s.