Defining classes

🔷 What is a Class in C++?

A class in C++ is a user-defined data type that acts as a blueprint for creating objects. It groups data (called data members) and functions (called member functions or methods) that operate on that data into a single unit.

Classes are the foundation of Object-Oriented Programming (OOP). They help in encapsulating (hiding) data, reusing code through inheritance, and modeling real-world entities.

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🔷 Syntax of a Class

class ClassName {
  private:
    // Private data members and functions

  public:
    // Public data members and functions

  protected:
    // Protected data members and functions (used in inheritance)
};

• private: Members cannot be accessed outside the class directly.
• public: Members can be accessed from outside the class using an object.
• protected: Used mainly in inheritance (not accessible directly outside the class, but accessible to derived classes).

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🔷 Example: Defining and Using a Class

#include <iostream>
using namespace std;

class Student {
  private:
    int rollNumber;
    float marks;

  public:
    string name;

    // Function to set private data
    void setData(int r, float m) {
        rollNumber = r;
        marks = m;
    }

    // Function to display student data
    void display() {
        cout << "Name: " << name << endl;
        cout << "Roll Number: " << rollNumber << endl;
        cout << "Marks: " << marks << endl;
    }
};

int main() {
    Student s1;
    s1.name = "Alice";
    s1.setData(101, 89.5);
    s1.display();

    return 0;
}

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🔷 Explanation of the Example

• Class Name: Student
• Data Members: rollNumber, marks, name
• Member Functions: setData(), display()
• Object: s1 is an object of the class Student
• The setData() function is used to initialize private data.
• The display() function shows student details.

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🔷 Key Concepts in Classes

Concept	Description
Encapsulation	Wrapping data and functions together. Keeps data safe from outside interference.
Data Members	Variables defined inside a class.
Member Functions	Functions defined inside a class.
Access Specifiers	Control how data/functions are accessed (public, private, protected).
Object	An instance of a class. Used to access class members.

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🔷 Memory Allocation

• Memory is allocated to class members only when an object is created.
• Each object gets its own copy of data members, but member functions are shared.

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🔷 Advantages of Using Classes

1. Modularity – Code is organized and modular.
2. Reusability – Classes can be reused using inheritance.
3. Security – Data hiding protects sensitive information.
4. Scalability – Easy to modify and scale programs.

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🔷 Diagram: Class vs Object (Text Form)

Class: Student
------------------------
| - rollNumber : int   |
| - marks : float      |
| + name : string      |
| + setData()          |
| + display()          |
------------------------

Object: s1
------------------------
| rollNumber = 101     |
| marks = 89.5         |
| name = "Alice"       |
------------------------

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🔷 Summary

• A class is a user-defined type in C++ that bundles data and functions.
• You use a class to create objects, and through these objects, you access the class members.
• Use public members to interact with the object and keep private members hidden for security and encapsulation.