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.

C++

#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;
}

Output

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.

Pure Virtual Functions and Abstract Classes in C++

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