Pure Virtual Functions

A pure virtual function is a virtual function that has no required implementation in the base class and must be implemented by concrete derived classes.

Syntax:

virtual return_type function_name(parameters) = 0;

Example:

virtual double area() const = 0;

What It Means

A pure virtual function says:

Every concrete child class must provide this behavior.

It creates a common interface.

Complete Example

#include <iostream>
using namespace std;

class Shape {
public:
    virtual ~Shape() = default;

    virtual double area() const = 0;
};

class Rectangle : public Shape {
private:
    double width;
    double height;

public:
    Rectangle(double w, double h) : width(w), height(h) {}

    double area() const override {
        return width * height;
    }
};

class Square : public Shape {
private:
    double side;

public:
    Square(double s) : side(s) {}

    double area() const override {
        return side * side;
    }
};

int main() {
    Rectangle rectangle(4, 5);
    Square square(3);

    Shape* shape = &rectangle;
    cout << shape->area() << endl;

    shape = &square;
    cout << shape->area() << endl;

    return 0;
}

Output:

20
9

Important Rule

If a class has at least one pure virtual function, it becomes an abstract class.

This is not allowed:

// Shape shape; // error: Shape is abstract

But this is allowed:

Shape* shape;
Shape& reference = rectangle;

You can use pointers and references to abstract classes.

Viva Answer

A pure virtual function is declared with = 0 and forces derived classes to implement it. A class with at least one pure virtual function becomes abstract and cannot be instantiated directly.

Quick Check

1. What does = 0 mean in a virtual function declaration?
2. Can we create an object of a class with a pure virtual function?
3. What happens if a derived class does not implement all pure virtual functions?