Question

Suppose a class Y publicly inherits a class X. Is it possible for a class Z to privately inherit Y while publicly inheriting X?

To make this clearer, suppose X defines public methods x1 and x2. Y inherits X, overrides x1 and provides a method y. Does C++ allow for a third class Z to subclass Y in such a way that Y‘s implementation of x1 and y are privately available to it, while the outside world only sees it inheriting X publicly, i.e. having only a single public method x2?

Answer 1

Yes, this is called virtual inheritance.

struct X {
  virtual void x1();
  void x2();
};

struct Y : virtual X {
  void x1(); // overrides
  void y();
};

struct Z : private Y, virtual X { 

};

People cannot do z.y() or z.x1() but they can do z.x2() and can convert a Z* to a X*. However once they do so, they can call converted->x1() and converted->x2(), of course.

You haven’t said anything about your goal, but it sounds like you really want to keep Y as a pointer though

struct X {
  virtual void x1();
  void x2();
};

struct Y : X {
  virtual void x1(); // overrides
  void y();
};

struct Z : X { 
  virtual void x1() { // overrides
    /* uses y->x1 */ 
  }
  Y *y;
};

This looks more familiar to me.