🟣 OOP · Lesson 49

Abstract Classes and Pure Virtual Functions

What is Abstract Classes and Pure Virtual Functions?

Abstract Classes and Pure Virtual Functions
An abstract class contains at least one pure virtual function and cannot be directly instantiated. It defines a common interface for derived classes.

Level
🟣 Object-Oriented Programming

Example File
abstract-classes.cpp

Main Focus
Concept + syntax + practical C++ program

Why should you learn this?

It helps you write correct and readable C++ programs.
It is used repeatedly in school practicals, projects and competitive programming.
It builds the base for advanced topics such as OOP, STL and data structures.

Important Terms

Term	Meaning / Use
abstract class	Abstract class used in Abstract Classes and Pure Virtual Functions programming.
pure virtual function	Pure virtual function used in Abstract Classes and Pure Virtual Functions programming.
interface	Interface used in Abstract Classes and Pure Virtual Functions programming.
override	Override used in Abstract Classes and Pure Virtual Functions programming.

Syntax / Pattern

virtual void show() = 0;

Example Program

#include <iostream>
using namespace std;
class Shape{ public: virtual double area() = 0; };
class Square: public Shape{ public: double area(){ return 25; } };
int main(){ Square s; cout << s.area(); }

Expected Output

Program Explanation

Shape has pure virtual area().
Square must implement area().

Exam Tip: In C++ practical answers, write the logic first, then the program, then expected output. For theory, always include one suitable example.

Where will you use it?

interfaces
framework design
common behavior rules

Common Mistakes

Trying to create object of abstract class.
Forgetting to override pure virtual function.

Practice Tasks

Create abstract class Payment.
Implement two derived classes.

Summary

Abstract Classes and Pure Virtual Functions is an important C++ topic. Learn the definition, understand the syntax, run the example program and then solve the practice tasks to make the concept strong.