class Circle
{
  public:
    Circle(double x, double y, double r);
      // no other Circle constructors are declared, so there's no default
      // constructor
    ...
  private:
    double m_x;
    double m_y;
    double m_r;
};

Circle::Circle(double x, double y, double r)
 : m_x(x), m_y(y), m_r(r)
{
    if (r <= 0)
    {
        ... write some error message ...
        exit(1);
    }
}

class StickFigure
{
  public:
    StickFigure(double bl, double headDiameter, string nm, double hx, double hy);
    ...
  private:
    string m_name;
    Circle m_head;
    double m_bodyLength;
};

StickFigure::StickFigure(double bl, double headDiameter, string nm,
							double hx, double hy)
 : m_name(nm), m_head(hx, hy, headDiameter/2), m_bodyLength(bl)
{
    if (bl <= 0)
    {
        ... write some error message ...
        exit(1);
    }
}