initializing default parameter with class member function/variable

Question

class C {
private:
     int n{ 5 };

public:
    int return5() { return 5; }
    void f(int d = return5()) {

    }

    void ff(int d = n) {

    }


};

Why I can't initialize the functions default parameters with member class? I get an error: a nonstatic member reference must be relative to a specific object.

I think the problem because no object has been instantiated yet, but is there any approach to do it?

Answer 1

The default argument is considered to be provided from the caller side context. It just doesn't know the object on which the non-static member function return5 could be called on.

You can make return5 a static member function, which doesn't require an object to be called on. E.g.

class C {
    ...
    static int return5() { return 5; }
    void f(int d = return5()) {
    }
    ...
};

Or make another overload function as

class C {
private:
     int n{ 5 };
public:
    int return5() { return 5; }
    void f(int d) {
    }
    void f() {
        f(return5());
    }
    void ff(int d) {
    }
    void ff() {
        ff(n);
    }
};