Classes and Objects

Classes and Objects

A class groups related data (member variables) and behavior (member functions) into a single unit. An object is an instance of a class.

Defining a Class

class Rectangle {
public:
    int width;
    int height;

    int area() {
        return width * height;
    }

    int perimeter() {
        return 2 * (width + height);
    }
};

• class — the keyword (vs C's struct)
• public: — these members are accessible from outside the class
• int area() — a member function that can access width and height directly

Creating Objects

Rectangle r;        // create an object
r.width = 10;       // set member variables
r.height = 5;
cout << r.area();   // call a member function → 50

Multiple Objects

Each object has its own copy of the member variables:

Rectangle a, b;
a.width = 3;  a.height = 4;
b.width = 6;  b.height = 2;

cout << a.area() << endl;  // 12
cout << b.area() << endl;  // 12

The this Pointer

Inside a member function, this is a pointer to the current object. You can write this->width to explicitly access a member, but it's optional — writing width alone works the same way. The this-> prefix is mainly useful when a parameter name shadows a member variable (e.g., this->width = width; in a constructor).

Member Functions Have Access to Members

Inside a member function, you can access any member variable or function of the same class:

class Circle {
public:
    double radius;

    double area() {
        return 3.14159 * radius * radius;
    }

    bool isLargerThan(Circle other) {
        return radius > other.radius;
    }
};

Class vs struct

In C++, class and struct are nearly identical. The only difference:

• struct members are public by default
• class members are private by default

Your Task

Define a Rectangle class with width and height (both int), plus area() and perimeter() methods. Create a rectangle with width 4 and height 3 and print its area and perimeter.