C++ Classes and Objects

🧱 C++ Classes and Objects

In C++, classes and objects are the foundation of Object-Oriented Programming (OOP).
A class defines a blueprint, and an object is a real instance created from that blueprint.

 1. What Is a Class?

A class is a user-defined data type that contains:

  • Data members (variables)

  • Member functions (methods)

class Student {
public:
int id;
string name;void display() {
cout << id << ” “ << name << endl;
}
};

 2. What Is an Object?

An object is an instance of a class.

int main() {
Student s1; // object creation
s1.id = 101;
s1.name = "Sanjit";
s1.display();
}

3. Multiple Objects

Student s1, s2;

s1.id = 101;
s1.name = “Amit”;

s2.id = 102;
s2.name = “Neha”;

Each object has its own copy of data members.

4. Access Specifiers in Class

public

Accessible everywhere.

private

Accessible only inside the class.

protected

Accessible in derived classes.

class Account {
private:
int balance;public:
void setBalance(int b) {
balance = b;
}

int getBalance() {
return balance;
}
};

5. Class with Constructor

class Student {
public:
int id;
string name;Student(int i, string n) {
id = i;
name = n;
}

void display() {
cout << id << ” “ << name;
}
};

Usage:

Student s1(101, "Sanjit");
s1.display();

6. Default Constructor

class Demo {
public:
Demo() {
cout << "Default Constructor Called";
}
};

7. Class Outside main()

class Box {
public:
int length;void set(int l) {
length = l;
}
};

8. Object with Pointer

Student s;
Student *p = &s;p->id = 101;
p->name = “Amit”;
p->display();

9. Array of Objects

Student s[2];

s[0].id = 1;
s[1].id = 2;

10. this Pointer

class Demo {
public:
int x;void set(int x) {
this->x = x;
}
};

11. Class vs Structure

ClassStructure
Members private by defaultMembers public by default
Used in OOPUsed for data grouping

📌 Summary

  • Class = blueprint

  • Object = instance of class

  • Classes contain data + methods

  • Access specifiers control visibility

  • Constructors initialize objects

  • Core concept of OOP in C++

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