Extended enum class

Question

I had enum class, say

enum class Enum{
   var1, var2;
}

Now I want to add some member which depends on parameter i.e var3(int). OK, It's not for enum, so I want to change it by regular class, but my goal is to leave old code(Enum::var1 as value of type Enum) possible to compile.

I tried to do it this way(let's temporary forgot about var3, it'll be static function):

class Enum{
    public:
        const static Enum var1 = Enum(1);
        const static Enum var2 = Enum(2);
    private:
        Enum(int v):v(v){
        }
    int v;
    //operator == using v
};

But it doesn't compile because Enum has incomplete type.
I can't declare it after class because it's in header so it'll not work with multiple cpp's. Besides, it's not great idea to have public constructor here.

Any thoughts?

Answer 1

Solution 1:

For the static variable problem: declare your static variables in the class declaration:

class Enum
{
    public:
        static const Enum var1;
        static const Enum var2;
        Enum(int v):v(v)
        {
        }
    private:
        int v;
        //operator == using v
};

Then, create a source file for this class, Enum.cpp, containing:

#include "Enum.h"
const Enum Enum::var1 = Enum(1);
const Enum Enum::var2 = Enum(2);

Solution 2:

If you want it to be header-only, you can use static variables instead of class variables:

class Enum
{
    public:
        Enum(int v):v(v)
        {
        }
    private:
        int v;
};

namespace Enumeration // It is not possible to name it 'Enum'
{
    // static => local to translation unit. No linking conflict
    static const Enum var1 = Enum(1);
    static const Enum var2 = Enum(2);
}

You can see a live example here. The only drawback is that you cannot use the name of the class for the namespace.

Answer 2

You can write a class like this:

class Enum
{
public
    enum NestedEnum : int
    {
        var1, var2
    };

    static NestedEnum var3;

    Enum(NestedEnum value) : value(value) { }

    operator NestedEnum() const { return value; }
private:
    NestedEnum value;
};

And anywhere else you can declare:

Enum::var3 = (Enum::NestedEnum)someIntegerVariable;