Pure Virtual Functions and Abstract Classes in C++

A pure virtual function is a virtual function with no implementation in the base class, declared using = 0. A class with at least one pure virtual function is an abstract class that cannot be instantiated and serves as a blueprint for derived classes, which must provide their own implementation.

• A class with at least one pure virtual function becomes an abstract class and Objects of abstract classes cannot be created directly.
• Abstract classes are used to define interfaces and ensure common structure among derived classes.
• Useful in polymorphism where different classes share the same interface but have different behaviors.
• A pure virtual function forces derived classes to override it.
• virtual void draw() = 0; declares a pure virtual function, forcing derived classes to provide their own implementation.

#include <iostream>
using namespace std;

class Shape {
public:
    virtual void draw() = 0; // Pure virtual function
};

class Circle : public Shape {
public:
    void draw() override {
        cout << "Drawing Circle\n";
    }
};

int main() {
    
    // Error in the below line : Cannot create 
    // object of abstract class
    // Shape s;      
    
    // Pointer to abstract class
    Shape* s = new Circle(); 
    
    // Output: Drawing Circle
    s->draw();    
    
    delete s;
}

Drawing Circle

Explanation: This example shows an abstract class Shape with a pure virtual function draw(). Circle overrides draw(), and using a Shape pointer to a Circle demonstrates polymorphism, outputting Drawing Circle.