Operator overloading of enum class in class

Question

I'm using private enum class in nested class and want to implement operator! for my enum class.

I know how to do this. But when I tried to overlord operator of enum class in nested class, compiler treats my operator as an operator of class, not for enum class.

class test{
    private:
        enum class Loc : bool{
            fwrd = true,
            bkrd = false
        };

        Loc Loc::operator!(){        //error msg 1.
             return Loc(!bool(*this));
        }

        Loc operator!(){
             return something;       //this treated as test's operator
        }


        Loc doSomething(Loc loc){
             return !loc;            //error msg 2.
        }


}

enum class Other : bool{
    fwrd = true,
    bkrd = false
};
Other operator!(Other o){                //this works
    return Other(!bool(*this));
}

Error msgs

"enum class test::Loc is not a class or a namespace.".
"no match for ‘operator!’ (operand type is ‘test::Loc’)"

@Jarod42 Oops! my mistake. edited

jumho park
– jumho park

2019-08-30 02:17:02 +00:00
Commented Aug 30, 2019 at 2:17 — jumho park
– jumho park, Commented Aug 30, 2019 at 2:17

Jarod42 · Accepted Answer · 2019-08-30 02:11:23Z

4

You might use friend functions:

class test
{
private:

    enum class Loc : bool{
        fwrd = true,
        bkrd = false
    };
    friend Loc operator!(Loc loc){
         return Loc(!bool(loc));
    }
    Loc doSomething(Loc loc){
         return !loc;
    }
};

Demo

answered Aug 30, 2019 at 2:11

Jarod42

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6 Comments

jumho park Over a year ago

wow it works... But i can't understand why 'friend' can solve this probelm. All that i know about 'friend' is 'friend function' or 'friend class', which giving access to others

M.M Over a year ago

It would be good to explain why friend makes a difference, rather than giving a code-only answer.

Red.Wave Over a year ago

enum cannot have member functions, so you gotta define the operator as a free function; but it has to access private member of class 'test', hence the freind declaration is needed. On the other hand for the sake of readability and avoiding dealing with probable namespace details, the function is defined within the class - resulting in ADL doing the rest.

jumho park Over a year ago

@Red.Wave what if i change accessibility of enum class to public? I've tested and it still needs friend .

Red.Wave Over a year ago

@jumhopark The free function('operator !' in this case) is normally defined at namespace scope. But when one defines a free function inside a class, there is no other way to inform the compiler that it is not a member; hence you need the 'freind' spec. Did you try defining the function outside the class?

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