Can I make a public member variable private in a derived class?

Question

I want to do make a public member in a base class private in a derived class, like this:

class A {
public:
    int   x;
    int   y;
};

class B : public A {
    // x is still public
private:
    // y is now private
    using y;
};

But apparently "using" can't be used that way. Is there any way to do this in C++?

(I can't use private inheritance because there are other members and functions of A that must still be public.)

Answer 1

Short answer: no. Liskov substitution and the nature of public inheritance demands that everything that you can do with an A (i.e. its public members) can also be done by B. That means you can't hide a public method.

If you're trying to hide public fields, there isn't much you can do. To "hide" public methods, you could do something like:

class B {
    // x is still public
    int x() { return a.x(); }
private:
    A a;
    // y is now private since you didn't add a forwarding method for it
};

Answer 2

Yes, using declaration technically allows you to do so.

You have to use using A::y instead of using y

However please seriously evaluate if doing this makes a design sense.

Few observations:

1. Your class should not have public data. That should be avoided as far as possible. If you stick to this design principle, you may not have a need to make it private in derived class.

2. Stick to LSP. If a base class has public method, and unless you are doing private inheritance, clients will be confused if the derived class makes the base class method private with such using declarations.