C++ Abstract Class and Pure Virtual Function

Before we learn about pure virtual functions, be sure to check these tutorials:

• C++ inheritance
• C++ virtual functions

C++ Pure Virtual Functions

Pure virtual functions are used

• if a function doesn't have any use in the base class
• but the function must be implemented by all its derived classes

Let's take an example,

Suppose, we have derived Triangle, Square and Circle classes from the Shape class, and we want to calculate the area of all these shapes.

In this case, we can create a pure virtual function named calculateArea() in the Shape. Since it's a pure virtual function, all derived classes Triangle, Square and Circle must include the calculateArea() function with implementation.

A pure virtual function doesn't have the function body and it must end with = 0. For example,

class Shape {
    public:

      // creating a pure virtual function
      virtual void calculateArea() = 0;
};

Note: The = 0 syntax doesn't mean we are assigning 0 to the function. It's just the way we define pure virtual functions.

Abstract Class

A class that contains a pure virtual function is known as an abstract class. In the above example, the class Shape is an abstract class.

We cannot create objects of an abstract class. However, we can derive classes from them, and use their data members and member functions (except pure virtual functions).

Example: C++ Abstract Class and Pure Virtual Function

// C++ program to calculate the area of a square and a circle

#include <iostream>
using namespace std;

// Abstract class
class Shape {
   protected:
    float dimension;

   public:
    void getDimension() {
        cin >> dimension;
    }

    // pure virtual Function
    virtual float calculateArea() = 0;
};

// Derived class
class Square : public Shape {
   public:
    float calculateArea() {
        return dimension * dimension;
    }
};

// Derived class
class Circle : public Shape {
   public:
    float calculateArea() {
        return 3.14 * dimension * dimension;
    }
};

int main() {
    Square square;
    Circle circle;

    cout << "Enter the length of the square: ";
    square.getDimension();
    cout << "Area of square: " << square.calculateArea() << endl;

    cout << "\nEnter radius of the circle: ";
    circle.getDimension();
    cout << "Area of circle: " << circle.calculateArea() << endl;

    return 0;
}

Output

Enter the length of the square: 4
Area of square: 16

Enter radius of the circle: 5
Area of circle: 78.5

In this program, virtual float calculateArea() = 0; inside the Shape class is a pure virtual function.

That's why we must provide the implementation of calculateArea() in both of our derived classes, or else we will get an error.