BCA - OOPS and Data Structures

A pure virtual function  is a special kind of function in C++ that: 

    Has no implementation  in the base class
    Forces derived classes to provide their own implementation.

We use pure virtual functions for: 
1. Creating Abstract Classes (Interfaces)  

An abstract class is a class that cannot be used to create objects directly . It exists only to be inherited by other classes. 

2. Enforcing a Common Interface  

When multiple classes inherit from a base class with pure virtual functions, they all have to implement those functions — ensuring a common structure .

Example :

#include <iostream>
using namespace std;

// Base class with pure virtual function
class Shape {
public:
    // Pure virtual function
    virtual float area() = 0;  // No body here
};

// Derived class - Circle
class Circle : public Shape {
private:
    float radius;

public:
    Circle(float r) {
        radius = r;
    }

    // Must override area()
    float area() override {
        return 3.14f * radius * radius;
    }
};

// Another derived class - Square
class Square : public Shape {
private:
    float side;

public:
    Square(float s) {
        side = s;
    }

    // Must override area()
    float area() override {
        return side * side;
    }
};

int main() {
    // Shape s; Error: Cannot create object of abstract class

    Circle c(5);
    Square s(4);

    cout << "Circle Area: " << c.area() << endl;   // Works
    cout << "Square Area: " << s.area() << endl;   // Works

    return 0;
}