Inheriting from an abstract class in CPP

Question

I've been learning about CPP inheritance and I cam across this question:

   Class polygon contains data member width and height and public method set_value() to assign values to width and height.
 Class Rectangle and Triangle are inherited from polygon class. 
the classes contain public method calculate_area() to calculate the area of Rectangle and Triangle.
 Use base class pointer to access the derived class object and show the area calculated. 

I wrote the following program for this problem:

#include <iostream>
using namespace std;
class Polygon{
    float height,width;
    public:
    void set_value(float height, float width){
        this->height=height;
        this->width=width;
    }
    virtual void calculate_area()=0;
};
class Rectangle:public Polygon{
    float height,width;
    public:
    void calculate_area(){
        cout << "Area of the rectangle is " << height*width <<endl;
    }
};
class Triangle:public Polygon{
    float height,width;
    public:
    void calculate_area(){
        cout << "Area of the triangle is " << 0.5*height * width << endl;
    }
};
int main(){
    Rectangle r1;
    r1.set_value(10,20);
    Triangle t1;
    t1.set_value(10,20);
    Polygon *p1;
    Polygon *p2;
    p1=&r1;
    p2=&t1;
    p1->calculate_area();
    p2->calculate_area();
    return 0;
}

However, on executing, it shows area of rectangle and triangle as zero. On further debugging, I found out that all dimensions (ie height and width of rectangle and triangle have value as zero). Can someone tell what's wrong with the code?

Answer 1

In your code Triangle and Rectangle inherit from Polygon , hence they also inherit Polygon s members height and width . Its those members that are used in Polygon s methods. Though they are private , hence cannot be accessed outside of Polygon .

Triangle and Rectangle have members height and width in addition to those inherited from Polygon . In Triangle and Rectangle s methods you are using those members. They are distinct from the members inherited from Polygon . Members of Triangle and Rectangle you do not initialize.

TL;DR: One possible fix is to grant derived classes access to the members by declaring them protected and remove the superflous definition of members in the derived classes:

#include <iostream>
using namespace std;
class Polygon{
    protected:
    float height,width;
    public:
    void set_value(float height, float width){
        this->height=height;
        this->width=width;
    }
    virtual void calculate_area()=0;
};
class Rectangle:public Polygon{
    public:
    void calculate_area() override {
        cout << "Area of the rectangle is " << height*width <<endl;
    }
};
class Triangle:public Polygon{    
    public:
    void calculate_area() override {
        cout << "Area of the triangle is " << 0.5*height * width << endl;
    }
};
int main(){
    Rectangle r1;
    r1.set_value(10,20);
    Triangle t1;
    t1.set_value(10,20);
    Polygon *p1;
    Polygon *p2;
    p1=&r1;
    p2=&t1;
    p1->calculate_area();
    p2->calculate_area();
    return 0;
}

Output :

Area of the rectangle is 200
Area of the triangle is 100

Note that I added override to the implementations of the virtual methods. When the overrides are ok, then it has not impact, but it will trigger a compiler error in case the method marked as override does actually not override (something that can happen easily by typos).