Different arguments to pure virtual functions c++

Question

I am working on an assignment about pure virtual functions. I have two different classes that i wish to use a pure virtual function. The virtual function is used to calculate area, each class i.e ( square, triangle) use different arguments to calculate the area. How can i make both getArea functions work with the pure virtual function?

#include <iostream>
#include <cmath>

using namespace std;

class Point{
    public:
    double x, y;
    Point(double a = 0, double b = 0){x = a; y = b;}
    Point operator + (Point const &obj){
        Point p;
        p.x = x + obj.x;
        p.y = y + obj.y;
        return p;
    }
     Point operator - (Point const &obj){
        Point p;
        p.x = x - obj.x;
        p.y = y - obj.y;
        return p;
    }
    friend ostream& operator<<(ostream& os, const Point& pt);

};

class Shape{
    public:
    virtual double getArea(Point a, Point b, Point c, Point d) = 0 ; // this is where i have a problem
    // need this to be virtual void getArea() = 0; so i can use it for every other class but not sure how
};

class Square: public Shape{
    public:
 // find area from four points

    double length(Point a, Point b){
        double hDis = pow((b.x - a.x),2);
        double vDis = pow((b.y - a.y),2);
        return sqrt(hDis + vDis);
        }
    double area_triangle(Point a, Point b, Point c){
        double A = length(a, b);
        double B = length (b, c);
        double C = length(a, c);
        double S = (length(a, b) + length (b, c) + length(a, c))/2;
        double area = sqrt((S*(S-A)*(S-B)*(S-C)));
        return area;
        }

    double getArea(Point a, Point b, Point c, Point d){ // have to calculate area with the point coordinates
        double area_tri1 = area_triangle(a, b, c);
        double area_tri2 = area_triangle(a, d, c);
        double total_area = area_tri1 + area_tri2;
        return total_area;
    }
};


class Triangle: public Shape{
    public:
double length(Point a, Point b){
        double hDis = pow((b.x - a.x),2);
        double vDis = pow((b.y - a.y),2);
        return sqrt(hDis + vDis);
        }
    double getArea(Point a, Point b, Point c){
        double A = length(a, b);
        double B = length (b, c);
        double C = length(a, c);
        double S = (length(a, b) + length (b, c) + length(a, c))/2;
        double air = sqrt((S*(S-A)*(S-B)*(S-C)));
        return air;
        }
};

ostream& operator<<(ostream& os, const Point& pt)
{
    os << pt.x << ", " << pt.y <<endl;
    return os;
}


int main(){

    Point p1(5,-5), p2(-10,7), p3(4, 23), p4(-6, 12);

    Square s;
    cout << s.getArea(p1, p2, p3, p4);


    //! Triangle t;
    //! cout << t.getArea(p1, p2,p3);

    // this gives me an error because the abstract function want
    // (Point a, Point b, Point c, Point d)
    // how do i make this work?
}

I think you are approaching the problem from the wrong direction. Every concrete child of shape should know how to calculate its area based on its member variables. No parameters should be required. — user4581301
– user4581301, Commented Dec 21, 2019 at 19:52
The Shape shouldn't know or care if what is derived needs 2, 3, 4, 5, or 100 points to calculate the area. Those items should be known by the derived class, not Shape. On the other hand, your design would be perfect if on purpose, you only wanted derived shapes that took exactly 4 points to define the area. The compiler did its job, and that is to error out because your Triangle violated the restrictions of how to compute the area. — PaulMcKenzie
– PaulMcKenzie, Commented Dec 21, 2019 at 19:59

Lightness Races in Orbit · Accepted Answer · 2019-12-21 19:53:40Z

4

You've designed your classes a bit strangely.

They should have data members ("member variables") that define their extents (as many Points as you need for that particular type).

Then there is no need to pass any arguments into getArea(): the implementation of this function in each class will use the member variables!

And then you won't have your problem any more, because all of the getArea() functions will have the same number of arguments: none.

If you move things out of function arguments, and into member variables, so that your classes actually contain some state, you should find that the rest of your design falls into place around that change.

answered Dec 21, 2019 at 19:53

Lightness Races in Orbit

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1 Comment

Peter Over a year ago

To put the data into members, there will also need some machinery in each concrete class to allow a user of the class to set those members. For example, Triangle might have a constructor that allows specifying the three vertices and some setter/getter capability.

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