Separating Enums from Class Definitions using Namespaces in C++?

Question

I'm working with a legacy class that looks like this:

class A
{
    enum Flags { One = 1, Two = 2 };
};

I'd like to pull out all the enums into a new namespace defined in a new header:

// flags.h

namespace flags {

enum Flags { One = 1, Two = 2 };

};

Then pull these enums back into the class so that I can include just the flags.h in headers that only require these values (rather than the entire class definition):

// a.h

#include "flags.h"

class A
{
    using namespace flags;
};

How should I be doing this?

Answer 1

class F {
  enum Flags { One = 1, Two = 2 };
};


class A : public F {
};

Answer 2

A using directive is illegal at class scope. Instead of a namespace you could define a class and then inherit from it:

struct flags {
    enum Flags { One=1, Two=2 };
};

class A : public flags { ... };

But this looks like a misuse of inheritance to me, to be honest. As an alternative you could bite the bullet and use a namespace without "importing" the names into class A.

Answer 3

You can't make them all visible in a class without pulling the values in seperately via using declarations.

The best thing, in my opinion, that you can do is wrapping them in a class that contains nothing else. That way you can make them visible in another class by deriving from the class that holds the enums:

struct Holder {
   enum E { a };
protected:
   Holder() {}
   ~Holder() {}
};

class User : public Holder 
{
public:
   void f() { /* a is visible here */ }
};

void f() { /* User::a is visible here */ }