Abstract Classes

What is an Abstract Class?

An abstract class in C++ is a class that contains at least one pure virtual function.
It cannot be instantiated directly, meaning you cannot create objects of an abstract class.

An abstract class serves as a base class and is meant to be inherited by other classes that will implement its pure virtual functions.

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🔑 Syntax of Abstract Class

cppCopyEditclass AbstractClass {
public:
    virtual void display() = 0;  // pure virtual function
};

Any class with = 0 in a virtual function becomes an abstract class.

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🧠 Real-Life Analogy

Imagine you have an abstract class called “Shape”.
You can’t draw “Shape” directly, but you can draw a Circle, Rectangle, or Triangle — each of which is a specific implementation of that abstract shape.

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✍️ Example of Abstract Class in 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" << endl;
    }
};

class Rectangle : public Shape {
public:
    void draw() override {
        cout << "Drawing Rectangle" << endl;
    }
};

int main() {
    Shape* s1 = new Circle();
    Shape* s2 = new Rectangle();

    s1->draw();  // Output: Drawing Circle
    s2->draw();  // Output: Drawing Rectangle

    delete s1;
    delete s2;

    return 0;
}

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📌 Key Points about Abstract Classes

Feature	Description
Has at least one pure virtual function	Yes (virtual func() = 0;)
Can be inherited	Yes
Can have normal functions	Yes
Can have data members	Yes
Can create object directly	❌ No (compilation error)
Can have constructors/destructors	✅ Yes

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🧾 Why Use Abstract Classes?

• To define a common interface for a group of classes.
• To implement runtime polymorphism.
• To enforce that derived classes must implement specific behaviors.
• To support the concept of “interface” or “blueprint” in OOP.

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❌ What Happens If You Don’t Override the Pure Virtual Function?

If a derived class does not override all the pure virtual functions from the abstract class, it also becomes abstract, and you cannot create objects of it.

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✅ Example: Error on Instantiating Abstract Class

class Base {
public:
    virtual void show() = 0;
};

int main() {
    Base b;  // ❌ Error: Cannot create object of abstract class
    return 0;
}

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🎯 Uses of Abstract Classes in Real Applications

• GUI applications: Abstract “Window”, “Button”, or “Event” classes
• File I/O operations: Abstract “FileHandler” class
• Games: Abstract “Character” or “Enemy” class
• Database systems: Abstract interface for query execution

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✅ Summary

Topic	Explanation
Abstract Class	A class with at least one pure virtual function
Purpose	Acts as a base class for derived classes
Object Creation	Not allowed
Used for	Interfaces, enforcing behavior, runtime polymorphism
Contains	Pure virtual functions, normal functions, and data members