C++: overriding public\private inheritance

Question

If B inherits from A using public, can B override one of the functions and force it to be private?

class A
{
public:
    virtual double my_func1(int i);
    virtual double my_func2(int i);
}

class B : public A // Notice the public inheritance
{
public:
    virtual double my_func1(int i);
private:
    virtual double my_func2(int i);
}

How about the other way around? if the inheritance type is private - can B force a specific function to be public?

What if A is pure abstract? does it make a difference?

Would protected make any difference in any combination?

Answer 1

If B inherits from A using public, can B override one of the functions and force it to be private? NO

Eventhough the my_func1() is declared under priavte access specifier it can be still called through a pointer to class A, actually pointing to a object of class B

The call to my_func1() is evaluated at run time depending on the type of objected pointed by the pointer. At compile time the compile sees the my_func1() call as call to A::my_func1() and since A::my_func1() is public the compiler doesn't report only error. It is only at runtime that actual function call B::my_func1() is evaluated.

Ofcourse, You cannot directly call my_func1() through object of class B though because B::my_func1() is declared under Private Access specifier and You cannot access privately declared members from outside the class.

How about the other way around? if the inheritance type is private - can B force a specific function to be public?
NO

If you are calling my_func1() through a pointer of the Base class A, At compile time it is just evaluated as call to A::my_func1() which is Invalid since A::my_func1() is declared private inclass A`

What if A is pure abstract? does it make a difference?
NO
It makes no difference if the base class is Abstract or just polymorphic. Same rules will be applicable.

Would protected make any difference in any combination?
NO
As explained in first 2 Q's if you are calling a virtual function thorough pointer to Base class then at compile time the compiler only checks the access of that member function in Base class because compiler sees it as call to Base class member function. The actual call to the function is evaluated at run time and the feature is called Runtime Polymorphism or Dynamic polymorphism which is independent of the Access specifiers, which as a compile time construct.

So in conclusion,

overriding members of Base Class does not affect access

Answer 2

Difference

What if A is pure abstract? does it make a difference?

The only difference it makes is the following, i.e how they can (or cannot) be used:

A *pa = new B();
pa->my_func2(10); //calls B::my_func2() even though its private!

B *pb = new B();
pb->my_func2(10); //compilation error - trying to access private function

Explanation

Access-specifiers are compile-time construct, and so, the compiler detects any violation of access-rules at compile-time (obviously) based on the static type of the object (or pointer). Such violation cannot be detected at runtime.

So pa->my_func2() works, because the compiler sees that the static type of pa is A* which has a public function my_func2() defined, so the expression pa->my_func2() passes the compiler's test. Hence it works.

But pb->my_func2() doesn't work, since the static type of pb is B* which has a private function my_func2(), hence the code wouldn't even compile!