Example: Circle Area using Class and Object in C++

// Circle Area using Class and Object
#include <iostream>
#include <cmath> // Required for M_PI constant

// Define PI if not available via cmath (e.g., older compilers or specific standards)
#ifndef M_PI
#define M_PI 3.14159265358979323846
#endif

// Define the Circle class
class Circle {
private:
    double radius; // Member variable to store the radius

public:
    // Constructor to initialize the radius
    Circle(double r) {
        radius = r;
    }

    // Member function to calculate the area of the circle
    double calculateArea() {
        return M_PI * radius * radius;
    }

    // Optional: Member function to display circle information
    void displayCircleInfo() {
        std::cout << "Radius: " << radius << std::endl;
        std::cout << "Area: " << calculateArea() << std::endl;
    }
    
    // Optional: Getter for radius
    double getRadius() {
        return radius;
    }
};

int main() {
    // Step 1: Declare a variable for the radius
    double r;

    // Step 2: Prompt the user to enter the radius
    std::cout << "Enter the radius of the circle: ";
    std::cin >> r;

    // Step 3: Validate the radius
    if (r < 0) {
        std::cout << "Radius cannot be negative. Please enter a positive value." << std::endl;
    } else {
        // Step 4: Create an object of the Circle class
        // The constructor Circle(r) is called to initialize the object's radius
        Circle myCircle(r);

        // Step 5: Call the calculateArea() member function using the object
        double area = myCircle.calculateArea();

        // Step 6: Display the result
        std::cout << "The area of the circle with radius " << myCircle.getRadius() << " is: " << area << std::endl;
        // Or using the displayCircleInfo method:
        // myCircle.displayCircleInfo();
    }

    return 0;
}