What is Meant by Pure Virtual Function in C++? | Scaler Topics

#include <iostream>  
using namespace std;  
// here is Abstract class  
class Shape  
{  
    public:  
    virtual float cal_Area() = 0; // cal_Area is a pure virtual function   
};  
class Square : public Shape  
{  
    float a;  
    public:  
    Square(float l)  
    {  
        a = l;  
    }  
    float cal_Area()  
    {  
        return a*a;    // returns area of square
    }  
};  
class Circle : public Shape  
{  
    float r;  
    public:  
      
    Circle(float x)  
    {  
        r = x;  
    }  
    float cal_Area()  
    {  
        return 3.14*r*r ;  
    }  
};  
class Rectangle : public Shape  
{  
    float l;  
    float b;  
    public:  
    Rectangle(float x, float y)  
    {  
       l=x;  
       b=y;  
    }  
    float cal_Area()  
    {  
        return l*b;  // returns the product of length and breadth
    }  
};  
int main()    // main function
{  
      
    Shape *shape;  
    Square s(3.4);  
    Rectangle r(5,6);  
    Circle c(7.8);  
    shape =&s;  
    int a1 =shape->cal_Area();  
    shape = &r;  
    int a2 = shape->cal_Area();  
    shape = &c;  
    int a3 = shape->cal_Area();  
    std::cout << "The area of square is: " <<a1<< std::endl;  
    std::cout << "The area of  rectangle is: " <<a2<< std::endl;  
    std::cout << "The area of  circle is: " <<a3<< std::endl;  
    return 0;  
}