Object-Oriented Programming (OOP) is one of the most important programming paradigms used in C++, Java, Python, and many modern languages. It organizes software design around objects rather than functions and logic.

In simple terms:

👉 OOP models real-world entities (objects) in programs.

For example:

• Car
• Bank Account
• User
• File
• Network Connection

Each object has:

• Data (attributes / properties)
• Behavior (methods / functions)

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1️⃣ What is Object-Oriented Programming?

Object-Oriented Programming (OOP) is a programming approach where software is structured using classes and objects.

Example idea:

Real world:

Car
 ├─ color
 ├─ speed
 └─ start()

In C++:

class Car
{
public:
    string color;
    int speed;    void start()
    {
        cout << "Car started\n";
    }
};

Object creation:

Car c1;
c1.color = "Red";
c1.speed = 100;
c1.start();

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2️⃣ Key Concepts of OOP

There are 4 main pillars of OOP.

Concept	Meaning
Encapsulation	Wrapping data and functions together
Abstraction	Showing only essential features
Inheritance	One class deriving from another
Polymorphism	Same function behaving differently

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3️⃣ Class and Object

Class

A class is a blueprint or template.

Example:

class Person
{
public:
    string name;
    int age;    void speak()
    {
        cout << "Hello\n";
    }
};

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Object

An object is an instance of a class.

Person p1;
p1.name = "Venkat";
p1.age = 30;
p1.speak();

Memory is allocated only when objects are created.

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4️⃣ Encapsulation

Encapsulation means:

👉 Binding data and functions together in one unit.

Also used for data protection.

Example:

class BankAccount
{
private:
    int balance;public:
    void deposit(int amount)
    {
        balance += amount;
    }    int getBalance()
    {
        return balance;
    }
};

Here:

• balance is private
• Access only through functions

Benefits:

• Data safety
• Controlled access

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5️⃣ Abstraction

Abstraction means:

👉 Hiding complex implementation details.

Example:

When you drive a car:

You use:

start()
brake()
accelerate()

You do NOT see:

• engine process
• fuel injection
• internal mechanics

C++ Example:

class Printer
{
public:
    void print()
    {
        cout << "Printing document\n";
    }
};

User only calls:

printer.print();

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6️⃣ Inheritance

Inheritance means:

👉 One class acquires properties of another class.

Example:

Parent class:

class Animal
{
public:
    void eat()
    {
        cout << "Animal eating\n";
    }
};

Child class:

class Dog : public Animal
{
public:
    void bark()
    {
        cout << "Dog barking\n";
    }
};

Usage:

Dog d;
d.eat();
d.bark();

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Types of Inheritance

Type	Description
Single	One parent → one child
Multiple	Multiple parents → one child
Multilevel	Grandparent → parent → child
Hierarchical	One parent → many children
Hybrid	Combination

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7️⃣ Polymorphism

Polymorphism means:

👉 Same function name, different behavior.

Two types:

Compile-time polymorphism

Function overloading

class Math
{
public:
    int add(int a, int b)
    {
        return a + b;
    }    double add(double a, double b)
    {
        return a + b;
    }
};

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Runtime polymorphism

Using virtual functions.

class Shape
{
public:
    virtual void draw()
    {
        cout << "Drawing shape\n";
    }
};class Circle : public Shape
{
public:
    void draw()
    {
        cout << "Drawing circle\n";
    }
};

Usage:

Shape* s;
Circle c;s = &c;
s->draw();

Output:

Drawing circle

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8️⃣ Constructors

Constructor initializes objects.

class Car
{
public:
    Car()
    {
        cout << "Car created\n";
    }
};

Runs automatically when object is created.

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9️⃣ Destructor

Destructor runs when object is destroyed.

class Car
{
public:
    ~Car()
    {
        cout << "Car destroyed\n";
    }
};

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🔟 Advantages of OOP

Advantage	Explanation
Code reuse	Using inheritance
Modularity	Programs divided into classes
Security	Data hiding
Easy maintenance	Small reusable modules
Scalability	Easy to extend systems

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1️⃣1️⃣ OOP in Large Systems

Used in:

• Operating systems
• Game engines
• Banking software
• Web servers
• Network systems
• GUI applications

Example C++ systems:

• Web servers
• Database engines
• Network frameworks

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1️⃣2️⃣ Simple Real World Example

Bank system design:

Classes:

Customer
Account
Transaction
ATM
Bank

Each object interacts with others.

Example:

Customer -> Account -> Transaction

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1️⃣3️⃣ Example Full Program

#include <iostream>
using namespace std;class Car
{
private:
    int speed;public:
    void setSpeed(int s)
    {
        speed = s;
    }    void showSpeed()
    {
        cout << "Speed: " << speed << endl;
    }
};int main()
{
    Car c1;    c1.setSpeed(120);
    c1.showSpeed();    return 0;
}

Output

Speed: 120

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⭐ Summary

Object-Oriented Programming is based on:

Class
Object
Encapsulation
Abstraction
Inheritance
Polymorphism

It helps build:

• Large systems
• Maintainable software
• Reusable code
• Secure programs