overriding with difference access specification c++

Question

I came across a question while taking iKM test. There was a base class with two abstract methods with private access specifier. There was a derived class which was overridin

Answer 1

Yes, this is legal, accessibility is checked statically (not dynamically):

class A {
public:
    virtual void foo() = 0;
private:
    virtual void bar() = 0;
};

class B : public A {
private:
    virtual void foo() {} // public in base, private in derived
public:
    virtual void bar() {} // private in base, public in derived
};

void f(A& a, B& b)
{
    a.foo(); // ok
    b.foo(); // error: B::foo is private
    a.bar(); // error: A::bar is private
    b.bar(); // ok (B::bar is public, even though A::bar is private)
}

int main()
{
    B b;
    f(b, b);
}

Now, why would you want to do that? It only matters if you use the derived class B directly (2nd param of f()) as opposed to through the base A interface (1st param of f()). If you always use the abstract A interface (as I would recommend in general), it still complies to the "IS-A" relashionship.