C++ program to define a class circle to represent circles. Add a data member radius to store the radius of a   circle. Write member functions area() and perimeter() to compute the area and perimeter of a circle.

    To define a Circle class in C++, we start by declaring the class with the class keyword, followed by the name of the class. In this case, we'll call it Circle. We'll then define a private data member to store the radius of the circle. Here's the code to do that:

  class Circle {
      private:
        double radius;
  };

       

    Next, we need to define public member functions that can set and get the value of the radius, as well as calculate the area and perimeter of the circle. Here's the full code for the Circle class:

    
class Circle {
  private:
    double radius;
  public:
    void setRadius(double r) {
        radius = r;
    }
    double getRadius() {
        return radius;
    }
    double area() {
        return 3.14159 * radius * radius;
    }
    double perimeter() {
        return 2 * 3.14159 * radius;
    }
};

       

As you can see, we've defined four public member functions for the Circle class:

• setRadius() sets the value of the radius to a given value.
• getRadius() returns the current value of the radius.
• area() calculates and returns the area of the circle using the formula πr².
• perimeter() calculates and returns the perimeter of the circle using the formula 2πr.

      

Using the Circle class to calculate area and perimeter

Now that we have defined the Circle class, we can use it to calculate the area and perimeter of a circle of a given radius. Here's the full code for a C++ program that uses the Circle class to calculate the area and perimeter of a circle with radius 5.0:

    
#include 
using namespace std;

class Circle {
  private:
    double radius;
  public:
    void setRadius(double r) {
        radius = r;
    }
    double getRadius() {
        return radius;
    }
    double area() {
        return 3.14159 * radius * radius;
    }
    double perimeter() {
        return 2 * 3.14159 * radius;
    }
};

int main() {
  Circle c;
  c.setRadius(5.0);
  cout << "Radius: " << c.getRadius() << endl;
  cout << "Area: " << c.area() << endl;
  cout << "Perimeter: " << c.perimeter() << endl;
  return 0;
}


       

    In this program, we first create an instance of the Circle class called c. We then set the radius of the circle to 5.0 using the setRadius() function. Finally, we use the getRadius(), area(), and perimeter() functions to print out the radius, area, and perimeter of the circle.