Classes

Introduction

In the previous lesson, you learned about pointers. So far, you have worked with variables, arrays, functions, strings, references, and pointers. These concepts allow programs to store data and perform operations, but larger applications need a better way to organize related information and behavior.

Consider a Student Management System. Each student may have a roll number, name, age, and marks. The system may also need to display student details, calculate percentages, and update records.

Student
├── Roll Number
├── Name
├── Age
├── Marks
├── Display Details
└── Calculate Percentage

Without classes, related data and functions may be scattered throughout the program. As the application grows, this makes the code harder to understand, maintain, and extend.

What is a Class?

A class is a user-defined type that can group data members and member functions into a single unit.

A class describes the structure and behavior that objects created from that class can have.

Class: Student
│
├── Data
│   ├── rollNumber
│   └── name
│
└── Behavior
    └── display()

Why Do We Need Classes?

Classes help organize programs around meaningful concepts. Instead of keeping related variables and functions separate, a class places them together under one type.

Real-World Analogy

Imagine an architect designing houses. Before construction begins, the architect creates a blueprint.

              📐 House Blueprint
                      │
          ┌───────────┼───────────┐
          │           │           │
          ▼           ▼           ▼
      🏠 House 1   🏠 House 2   🏠 House 3

The blueprint is not an actual house. It describes the structure that houses created from it should follow.

Similarly, a class defines a type, while objects are instances created from that type.

Characteristics of a Class

Class Structure

A class commonly contains data members and member functions.

class Student
{
public:
    // Data Members
    int rollNumber;
    std::string name;

    // Member Function
    void display()
    {
        // Behavior
    }
};

General Syntax

class ClassName
{
public:
    // Data Members

    // Member Functions
};

class Student
{
public:
    int rollNumber;
    std::string name;

    void display()
    {
        // Function body
    }
};

Example 1: Creating a Class

#include <iostream>
#include <string>

class Student
{
public:
    int rollNumber;
    std::string name;
};

int main()
{
    Student student1;

    student1.rollNumber = 101;
    student1.name = "Rahul";

    std::cout << student1.rollNumber << std::endl;
    std::cout << student1.name;

    return 0;
}

101
Rahul

student1
┌──────────────────────┐
│ rollNumber = 101     │
│ name       = Rahul   │
└──────────────────────┘

Accessing Members

For an object, accessible non-static members can be selected using the dot operator.

object.member

student1.rollNumber;
student1.name;
student1.display();

Example 2: Member Function

A class can contain functions in addition to data. Functions associated with a class are called member functions.

#include <iostream>

class Student
{
public:
    void display()
    {
        std::cout << "Welcome to C++";
    }
};

int main()
{
    Student student1;

    student1.display();

    return 0;
}

Welcome to C++

Example 3: Data Members and Member Function Together

Member functions can work with the data members of the object for which they are called.

#include <iostream>
#include <string>

class Student
{
public:
    int rollNumber;
    std::string name;

    void display()
    {
        std::cout << "Roll Number : "
                  << rollNumber << std::endl;

        std::cout << "Name : "
                  << name;
    }
};

int main()
{
    Student student1;

    student1.rollNumber = 101;
    student1.name = "Rahul";

    student1.display();

    return 0;
}

Roll Number : 101
Name : Rahul

student1
┌────────────────────────┐
│ rollNumber = 101       │
│ name       = Rahul     │
│                        │
│ display()              │
└────────────────────────┘
          │
          ▼
Displays student1's data

Memory Representation

Each object has its own state represented by its non-static data members.

Student student1;
Student student2;

student1.rollNumber = 101;
student1.name = "Rahul";

student2.rollNumber = 102;
student2.name = "Priya";

student1
┌──────────────────────┐
│ rollNumber = 101     │
│ name       = Rahul   │
└──────────────────────┘

student2
┌──────────────────────┐
│ rollNumber = 102     │
│ name       = Priya   │
└──────────────────────┘

Changing the state of student1 does not automatically change the state of student2. They are separate objects.

Program Execution Flow

The class defines the type. Objects are then created from that type, given their own state, and used to access members or call member functions.

Class vs Object

A class and an object are related, but they are not the same thing.

class Student
{
public:
    int rollNumber;
};

// Objects
Student student1;
Student student2;
Student student3;

Student Class
     │
     ├────► student1
     ├────► student2
     └────► student3

Real-World Applications

Classes can model entities and concepts in many kinds of software.

E-Commerce Application
│
├── Product
├── Customer
├── Cart
├── Order
└── Payment

Advantages of Classes

Common Beginner Mistakes

// ❌ Incorrect
class Student
{
}

// ✅ Correct
class Student
{
};

class Student
{
public:
    std::string name;
};

Student student1;

// ❌ Incorrect for a non-static member
// Student.name;

// ✅ Correct
student1.name = "Rahul";

class Student
{
    int rollNumber; // private by default
};

class Student
{
public:
    int rollNumber;
};

Best Practices

class BankAccount
{
    // Account-related members
};

class Student
{
    // Student-related state
    // Student-related behavior
};

Frequently Asked Questions

Key Takeaways

Summary

Classes allow C++ programs to define new types that combine related data and behavior.

A class can contain data members that represent state and member functions that define operations. Objects are instances created from the class type, and each object can maintain its own state.

Accessible members can be selected through an object using the dot operator. Member functions can work directly with the data belonging to the object for which they are called.

Classes improve organization, provide controlled access to implementation details, support reusable types, and form an important foundation for object-oriented programming.

Now that you understand what a class defines, the next lesson will focus more deeply on objects: how they are created, how multiple objects maintain independent state, and how object lifetime works.