Question

I have two questions.

In C++, a static member function has direct access to a public non-static data member defined in the same class?
False

In C++, a non-static member function has direct access to a private static data member defined in the same class?
True

My note say false for the first question and true for the second one. I just cannot find out why? Can you explain why this is? Thank you.

P.S. I’m studying for my final and I cannot seem to figure out why.

Answer 1

Everyone’s in agreement, but should be very careful about their wording, because actually static member functions do have access to public non-static data members. For that matter, they have access to private non-static data members too. They just need an object to operate on, to access its members. This could be a parameter, or a global, or created in the static member function, or acquired via one of those things.

The following code is fine:

class foo {
public:
    int a;
    // static member function "get_a" ...
    static int get_a(foo *f) {
        // ... accesses public non-static data member "a"
        return f->a;
    }
};

So we ask ourselves, what’s the difference between “access” and “direct access”?

I guess what’s meant by “direct access” here must be “using only the name of the data member, without specifying an object”. Everyone always needs to have an object in order to access non-static members – that’s what non-static means. Non-static member functions just don’t have to mention which object if they don’t want to, because this is implicit. Hence their access to non-static data members can be direct.

The reason non-static member functions have direct access to private static data members is firstly that the code is in a member of the class, hence it can access private data members. Second, you never need an object in order to access static data members (you can specify one if you want, but all that’s used is the static type of the expression, not the actual object), hence the access is direct.