Question

Let’s assume I want a single class Child and only this class to inherit from Father
and grant Child access to Fathers data members. Apart from this no one should inherit from father to get to its internals, hence I keep the data of Father private

Edit: I opted against protected since I want the data hidden. Some people advocate this as good practice but perhaps its a little too dogmatic: (e.g. Herb Sutter in http://drdobbs.com/184403867) Actually the class should be extended only once. Hence prohibiting inheritance as Steve Jessop pointed out would be an option but I think the cost of that (virtual etc.) is going too far for what I need.

Code would look like this:

class Father{
  friend class Child;
  public: 
    //functions
  private:
    int mData;
}

class Child: public Father{
  public:
    void changeData(int val){mData=val;}
}

Am I running into some serious trouble here or is this a valid decision assuming that inheritance was a good decision in the first place.

Answer 1

This doesn’t actually prevent anyone else using Father as a base class, it just prevents them from accessing mData if they do.

If that’s what you want (Child has privileged access to Father, that other classes don’t have, and the fact that Child happens to also have a base class Father is unrelated), fine.

If you additionally forbid anyone else from using Father as a base class (either via documentation alone, or using the trick with the virtual base class), then the use of friend becomes a bit pointless. You might as well just make mData protected instead of private, that’s exactly what protected is for.