虚拟继承中的重载虚拟函数

Question

My question is bit lengthy. Kindly answer it only once you go through the whole problem.

I have implemented the Diamond Problem as follows:

class Polygon
{

protected:
    int sides;

public:

    Polygon()
    {
        cout << "Polygon's Default Constructor being called." << endl;
    }

    Polygon(int a)
    {
        cout << "Polygon's parameterized Constructor being called." << endl;
        sides = a;
    }

    void virtual Draw()
    {
        cout << "Polygon being drawn." << endl;
    }

    virtual ~Polygon()
    {
        cout << "Polygon's Destructor being called." << endl;
    }

};


class Triangle : virtual public Polygon
{
    int Angles[3];
public:
    Triangle()
    {
        cout << "Triangle's Default Constructor being called." << endl;

    }

    Triangle(int a)
    {
        cout << "Triangle's parameterized Constructor being called." << endl;
        sides = a;
    }

    Triangle(int a, int b) : Polygon(a)
    {
        cout << "Triangle's double parameterized Constructor being called." << endl;
        //sides = a;
    }


    void virtual Draw()
    {
        cout << "Triangle being drawn." << endl;
    }

    ~Triangle()
    {
        cout << "Triangle's Destructor being called." << endl;
    }

};

class IsoscelesPolygon : virtual public Polygon
{
    void virtual Draw()
    {
        cout << "Isosceles Polygon's Draw Called." << endl;
    }
};

class IsoscelesTriangle : public Triangle, public IsoscelesPolygon
{

    void Draw(int )
    {
        cout << "Isoceles Triangle's Draw() Called." << endl;
    }
};

It works perfectly fine and resolves the Diamond Problem due to virtual inheritance. But when I change Draw() in IsocelesTriangle to Draw(int) , it starts giving me error like this:

This error doesn't pop up and program successfully runs (in non-polymorphic way obviously) when I make the Draw() in Polygon as non-virtual. Why? What link does it ( virtual function in base class) have to do with the signature of Draw() in IsocelesTriangle ?

Answer 1

I believe the idea is that if you don't override Draw in IsoscelesTriangle (and changing the signature is not overriding anymore), you end up with 2 Draw functions in the final class IsoscelesTriangle , one from IsoscelesPolygon and other from Triangle , and both try to override the Draw from Polygon . The compiler finds it ambiguous. Note that g++ spits out a more readable error:

error: virtual function 'Polygon::Draw' has more than one final overrider in 'IsoscelesTriangle'

The virtual inheritance just ensures that the base object Polygon is not appearing twice in IsoscelesTriangle . In your case, whenever you explicitly override Draw , the compiler hides the other 2 Draw s coming from Triangle and IsoscelesPolygon , so no more confusion.

PS: you'll find the same error even if you remove Draw completely from IsoscelesTriangle . Good question anyway, hope I got it right.

---

Now, regarding the last part of your question

This error doesn't pop up and program successfully runs (in non-polymorphic way obviously) when I make the Draw() in Polygon as non-virtual

The idea here is that now each Triangle and IoscelesPolygon declare Draw as virtual , so basically they start with a clean state and overload the function Draw from Polygon (which was marked as non-virtual). You then end up with 2 different Draw functions in IsoscelesTriangle , which are not trying to override the Draw from Polygon .