Classes & Objects in C++: Fundamentals & First OOP Program

1. What is a Class and What is an Object? (In Simple Words)

In traditional C programming, data and functions were kept separate. Anyone could change global variables by mistake, which caused bugs.

Object-Oriented Programming (OOP) fixes this by wrapping data (variables) and the functions that work on that data together into one secure package.

Real-Life Examples to Remember for Exams:

1. Architect Blueprint vs. House: An architect draws a 3BHK house plan on paper — that is a Class (takes 0 land). The actual house built with bricks and cement in your city is an Object (takes real land!). You can build 50 identical houses from 1 blueprint.
2. Car Blueprint vs. Car: The engineering design of a Maruti Swift is the Class. The actual car standing in your college parking lot is an Object.

2. Syntax of a Class in C++

To create a class, use the class keyword followed by the class name. The body is wrapped inside curly braces {} and must end with a semicolon ;.

class Student {
    private:
        // Variables (Data Members)
        // Hidden from outside world (Data Hiding)
        int rollNo;

    public:
        // Functions (Member Functions / Methods)
        // Can be called from main() to use the object
        void input();
        void display();
}; // Note the mandatory semicolon!

3. Access Specifiers: private vs. public vs. protected

Access specifiers decide who is allowed to view or use the members of a class:

Exam Tip: In C++, if you don’t write anything, all members are private by default. (In a C++ struct, members are public by default).

4. Your First Complete OOP Program in C++

Here is a complete, working student management program:

#include <iostream>
#include <string>
using namespace std;

// 1. Defining the Class Blueprint
class Student {
    private:
        // Private Data: Protected from outside tampering
        int rollNumber;
        string name;
        float marks;

    public:
        // Public Member Function 1: Take input from user
        void getDetails() {
            cout << "Enter Roll Number: ";
            cin >> rollNumber;
            cin.ignore(); // Clears enter key from input buffer
            cout << "Enter Student Name: ";
            getline(cin, name);
            cout << "Enter Marks: ";
            cin >> marks;
        }

        // Public Member Function 2: Display student report
        void displayDetails() {
            cout << "\n--- Student Report Card ---" << endl;
            cout << "Roll No : " << rollNumber << endl;
            cout << "Name    : " << name << endl;
            cout << "Marks   : " << marks << endl;
            cout << "Result  : " << (marks >= 40 ? "PASSED" : "FAILED") << endl;
        }
}; // End of Class with semicolon

int main() {
    // 2. Creating two Objects (Instances) of class Student
    Student s1;
    Student s2;

    cout << "=== Entering Data for Student 1 ===" << endl;
    s1.getDetails(); // Calling public function using dot (.) operator

    cout << "\n=== Entering Data for Student 2 ===" << endl;
    s2.getDetails();

    // Displaying data for both students
    s1.displayDetails();
    s2.displayDetails();

    // The line below gives a COMPILER ERROR because rollNumber is private!
    // s1.rollNumber = 101; 

    return 0;
}

5. The Dot Operator (.)

To call a public function or access a public variable of an object inside main(), we use the Dot Operator (.):

objectName.functionName();

For example: s1.getDetails(); calls getDetails() specifically for object s1.

DAVV Exam Preparation Tips

Frequently Asked 5-Mark Questions:

1. Differentiate between a Class and an Object with an example.
   Model Answer:
   • Class: A user-defined data type and blueprint that defines properties and behaviors. Takes 0 bytes of RAM.
   • Object: A physical instance of a class that occupies memory in RAM.
   • Give the Blueprint vs. House or Car design vs. Real car example.
2. What are access specifiers in C++? What is the default access specifier?
   Model Answer:
   • Explain private, public, and protected.
   • Mention that in C++ classes, members are private by default.

Frequently Asked 10-Mark Questions:

1. Explain the fundamental concepts of OOP: Encapsulation, Data Hiding, and Abstraction. Write a complete C++ program to create a BankAccount class with account number, customer name, and balance, with deposit, withdraw, and check-balance methods.
   Model Answer:
   • Define Encapsulation (wrapping data and methods into a single unit).
   • Define Data Hiding (making balance private so outside code cannot tamper with money directly).
   • Write out the complete C++ class BankAccount with deposit(), withdraw() (check if amount > balance), and checkBalance().
   • Create objects in main() and demonstrate successful transactions.