// Circle.h

#ifndef CIRCLE_INCLUDED
#define CIRCLE_INCLUDED

class Circle
{
  public:
    Circle(double x, double y, double r);
    void draw() const;
    bool scale(double factor);
    double radius() const;
  private:
    double m_x;
    double m_y;
    double m_r;
      // Class invariant:  m_r > 0
};

double area(const Circle& circ);

#endif  // CIRCLE_INCLUDED

=======================================================

// Circle.cpp

#include "Circle.h"
#include <iostream>
#include <cstdlib>
#include <cmath>
using namespace std;

Circle::Circle(double x, double y, double r)
 : m_x(x), m_y(y), m_r(r)
{
    if (r <= 0)
    {
        cerr << "Cannot create a circle of radius " << r << endl;
        exit(1);
    }
}

void Circle::draw() const
{
    ... draw the circle ...
}

bool Circle::scale(double factor)
{
    if (factor <= 0)
        return false;
    m_r *= factor;
    return true;
}

double Circle::radius() const
{
    return m_r;
}

double area(const Circle& circ)
{
    const double PI = 4 * atan(1.0);
    return PI * circ.radius() * circ.radius();
}

========================================================

// myapp.cpp

#include "Circle.h"
#include <iostream>
using namespace std;

int main()
{
    Circle c(-2, 5, 10);
    c.scale(2);
    c.draw();
    cout << area(c);
}