Question

I was going over some sample questions for an upcoming test, and this question is totally confusing me.

Consider the following code:

class GraduateStudent : public Student 
{
  ...
};

If the word “public” is omitted, GraduateStudent uses private inheritance, which means which of the following?

1. GraduateStudent objects may not use methods of Student.

2. GraduateStudent does not have access to private objects of Student.

3. No method of GraduateStudent may call a method of Student.

4. Only const methods of GraduateStudent can call methods of Student.

Answer 1

Although this is a bare homework-ish question, I’m going to answer it because it is a terrible question. I would almost consider it a trick question, and it doesn’t really make for a good test of knowledge.

The answer is 2. GraduateStudent does not have access to private objects of Student., except that this has nothing at all to do with private inheritance. Point 2 would be true whether or not the public keyword were present, since derived classes never have access to the private members of their base classes, no matter how they inherit.

Private inheritance means essentially two things (as opposed to public inheritance):

1. All public methods of Student become private methods in GraduateStudent. That means that if, for example, Student has a public method foo(), then GraduateStudent has a private method foo().

2. The base class is “inaccessible”, which means that polymorphism does not work. In layman’s terms, this means that if GraduateStudent inherits privately from Student, then you cannot treat a GraduateStudent* as if it were a Student* (or a GraduateStudent& as if it were a Student&).

It’s possible that the author of the question also meant for point 1 to be a correct answer, but it is ambiguously worded. What does it mean that “GraduateStudent objects may not use methods of Student“? It’s possible that the intent is for this to mean that you cannot call methods inherited from Student on objects of type GraduateStudent, like I wrote in the first point above, but the GraduateStudent object itself, within its methods, can use methods of Student.

For example:

class Student {
  public: 
    void foo() {};
};

class GraduateStudent : Student {
  public: 
   void bar()
   {
     foo(); // Legal
   }
};

int main() {
  GraduateStudent g;

  g.bar(); // Legal
  g.foo(); // Illegal

  return 0;
};