Using Base pointers to call derived object function

Question

I need to be able to use a base pointer to hold the address of either a rectangle or a circle. Which one would be determined at run time. I then want to use the pointers to call different virtual functions depending on which type they are. I can get this concept to work if the functions only use one Shape pointer. However many of my functions require two objects to work.

If I use a pure virtual functions, both Rectangle and Circle classes become abstract and I can not use the objects (error C2259). If I declare the functions as I did down below, all of the calls go to the Base class Shape. Any help is greatly appreciated.

class Shape {
public:
    virtual double overlappingArea(const Shape&)const {return 0;};
    //replacing with a pure virtual function causes the other classes to become abstract
    //virtual double overlappingArea(const Shape&)const = 0;
    //This returns error C2259 (or pure virtual function has no overload)
    //I know this is because the program has no overloads with identical parameters

};

class Rectangle : virtual public Shape {
public:
    Rectangle(int X, int Y, int L, int W) : x(X), y(Y), l(L), w(W) {}

    double overlappingArea(const Rectangle& R)const {
        double area = 1.1;
        //code that finds the overlapping area
        return area;
    }

    double overlappingArea(const Circle& C)const {
        double area = 1.2;
        //code that finds the overlapping area
        return area;
    }
private:
    int x, y, l, w;
};

class Circle: virtual public Shape {
public:
    Circle(int X, int Y, int R) : x(X), y(Y), r(R) {}

    double overlappingArea(const Rectangle& R)const {
        double area = 2.1;
        //code that finds the overlapping area
        return area;
    }

    double overlappingArea(const Circle& C)const {
        double area = 2.2;
        //code that finds the overlapping area
        return area;
    }
private:
    int x, y, r;
};

int main() {
    Shape* F1 = new Rectangle(0,0,1,1);
    Shape* F2 = new Rectangle(1,1,2,2);
    Shape* C1 = new Circle(0,0,1);
    Shape* C2 = new Circle(1,1,2);

    double areaFF, areaFC, areaCC;

    areaFF = F1->overlappingArea(*F2);
    areaFC = F1->overlappingArea(*C1);
    areaCC = C1->overlappingArea(*C2);    

    return 0;
}

All the areas end up equaling 0. I want areaFF = 1.1 , areaFC = 1.2 , areaCC = 2.2

Thanks for the help
Working code if you're interested

#include <iostream>
using namespace std;

class Rectangle;
class Circle;

class Shape {
public:
    virtual double overlapwith(const Shape&)const = 0;
    virtual double overlap(const Rectangle&)const = 0;
    virtual double overlap(const Circle&)const = 0;

};


class Circle : public Shape {
public:
    Circle() : x(0), y(0), r(0) {
    }
    Circle(int X, int Y, int R) : x(X), y(Y), r(R) {
    }

    double overlapwith(const Shape &with)const {
        cout << "\nCirc::overlapwith(const Shap&)const";
        return with.overlap(*this);
    }
    double overlap(const Rectangle &w)const {
        cout << "\nCirc::overlap(const Rect&)const";
        return 12;
    }
    double overlap(const Circle &w)const {
        cout << "\nCirc::overlap(const Circ&)const";
        return 11;
    }

private:
    int x, y, r;

};

class Rectangle : public Shape {
public:
    Rectangle() : x(0), y(0), l(0), w(0) {
    }
    Rectangle(int X, int Y, int L, int W) : x(X), y(Y), l(L), w(W) {
    }

    double overlapwith(const Shape &with)const {
        cout << "\nRect::overlapwith(const Shap&)const";
        return with.overlap(*this);
    }
    double overlap(const Rectangle &w)const {
        cout << "\nRect::overlap(const Rect&)const";
        return 22;
    }
    double overlap(const Circle &w)const {
        cout << "\nRect::overlap(const Circ&)const";
        return 21;
    }

private:
    int x, y, l, w;

};





int main() {

    Shape* F1 = new Rectangle(0,0,1,1);
    Shape* F2 = new Rectangle(1,1,2,2);
    Shape* C1 = new Circle(0,0,1);
    Shape* C2 = new Circle(1,1,2);


    double ff, fc, cf, cc;
    ff = F1->overlapwith(*F2);
    fc = F1->overlapwith(*C2);
    cf = C1->overlapwith(*F2);
    cc = C1->overlapwith(*C2);

    cout << "\n\n\tff : " << ff
        << "\n\tfc : " << fc
        << "\n\tcf : " << cf
        << "\n\tcc : " << cc;

    int pb; cin >> pb;
    return 0;
}

Start by removing class-name qualification inside the class (eg. Circle::), and look up the keyword 'override', if you're using C++11. — Shaggi
– Shaggi, Commented Dec 5, 2016 at 3:04
Sorry about the Rectangle:: . I copied some of the code from a program where each class had a separate .h and .cpp file — julian kizanis
– julian kizanis, Commented Dec 5, 2016 at 3:10

Sam Varshavchik · Accepted Answer · 2016-12-05 03:11:27Z

1

Define two more pure virtual methods in the base class, in addition to the existing virtual methods, which can be pure. You will need to do some simple forward declarations:

class Rectangle;
class Circle;

class Shape {
public:
    virtual double overlappingArea(const Shape&) const=0;
    virtual double overlappingAreaWith(const Rectangle&) const=0;
    virtual double overlappingAreaWith(const Circle&) const=0;
};

In each subclass, implement the first virtual method (the existing one) by invoking overlappingAreaWith() for the passed Shape & parameter, passing *this as the parameter:

class Rectangle {

    // ...

    double overlappingArea(const Shape &with) const override
    {
        return with.overlappingAreaWith(*this);
    }
};

class Circle {

    // ...

    double overlappingArea(const Shape &with) const override
    {
        return with.overlappingAreaWith(*this);
    }
};

Now, implement the other two overlappingAreaWith() methods in both Circle and Rectangle subclasses. They will now receive the other object, as either a Circle or a Rectangle parameter, as the case may be.

Each subclass is correctly implementing all three pure virtual methods.

answered Dec 5, 2016 at 3:11

Sam Varshavchik

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1 Comment

Quentin Over a year ago

For the record, this is the Visitor pattern.

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