Pure Virtual Functions and Abstract Classes in C++

What is an Abstract Class in C++?

An abstract class is a class that cannot be used to create objects directly. It is meant to be a base class that provides a common interface but doesn't define full behavior.

A class becomes abstract if it has at least one pure virtual function.

What is a Pure Virtual Function?

A pure virtual function is a function declared in a base class that must be overridden in any derived class.

It is written like this:

virtual void draw() = 0;  // Pure virtual function

• The = 0 means “no default implementation”.
• It forces derived classes to implement the function.

Example of Abstract Class and Pure Virtual Function

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

Abstract Class Vs Interface

Summary

• An abstract class defines what derived classes must do, but not how.
• A pure virtual function is a function with = 0 and must be overridden.
• You can't create objects of abstract classes.
• Useful when you want to define a common interface but let derived classes define specific behavior.