Classes and Objects in C++

A class is a user-defined data type that acts as a blueprint for creating objects. It bundles data (attributes) and functions (methods) that operate on that data into one unit. An object is a specific instance of a class — created from the blueprint.

Defining a Class

class ClassName {
public:
    // data members (attributes)
    // member functions (methods)
};

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

class Dog {
public:
    string name;
    string breed;
    int age;

    void bark() {
        cout << name << " says: Woof! Woof!" << endl;
    }
};

Creating Objects

int main() {
    Dog dog1;
    dog1.name  = "Bruno";
    dog1.breed = "Labrador";
    dog1.age   = 3;
    dog1.bark();

    Dog dog2;
    dog2.name  = "Milo";
    dog2.breed = "Poodle";
    dog2.age   = 2;
    dog2.bark();
    return 0;
}

Output:

Bruno says: Woof! Woof!
Milo says: Woof! Woof!

Access Specifiers

Access specifiers define who can access the members of a class:

Private Members and Getter/Setter Functions

Making data private and providing controlled access through public functions is the essence of encapsulation:

class BankAccount {
private:
    double balance;

public:
    void setBalance(double amount) {
        if (amount >= 0) balance = amount;
    }

    double getBalance() {
        return balance;
    }
};

int main() {
    BankAccount acc;
    acc.setBalance(5000);
    cout << "Balance: " << acc.getBalance() << endl;
    return 0;
}

Output:

Balance: 5000

Member Functions Defined Outside the Class

Member functions can be defined outside the class using the scope resolution operator :::

class Rectangle {
public:
    double length;
    double width;
    double area();   // declaration
};

double Rectangle::area() {   // definition outside
    return length * width;
}

int main() {
    Rectangle r;
    r.length = 6.0;
    r.width  = 4.0;
    cout << "Area = " << r.area() << endl;
    return 0;
}

Output:

Area = 24

The this Pointer

Inside any member function, the this pointer refers to the current object. It is useful when parameter names conflict with member names:

class Circle {
private:
    double radius;

public:
    void setRadius(double radius) {
        this->radius = radius;   // 'this' resolves ambiguity
    }

    double getArea() {
        return 3.14 * radius * radius;
    }
};

Array of Objects

class Point {
public:
    int x, y;
    void show() {
        cout << "(" << x << ", " << y << ")" << endl;
    }
};

int main() {
    Point pts[3] = {{1,2}, {3,4}, {5,6}};
    for (int i = 0; i < 3; i++) {
        pts[i].show();
    }
    return 0;
}

Output:

(1, 2)
(3, 4)
(5, 6)

Objects as Function Arguments

class Box {
public:
    int volume;
};

void displayVolume(Box b) {
    cout << "Volume: " << b.volume << endl;
}

int main() {
    Box b;
    b.volume = 120;
    displayVolume(b);
    return 0;
}

Key Takeaways

• A class is a blueprint; objects are created from it.
• Class members can be public, private, or protected.
• Private data members should be accessed via public getter and setter functions.
• The scope resolution operator :: is used to define member functions outside the class.
• The this pointer refers to the current object inside member functions.