class Shape // an abstract class
{
public:
   Shape();
private:
   void init();
   virtual void reset() = 0;
   int _color;
};

Shape::Shape() { init(); }
void Shape::init() { reset(); }

class Point : public Shape // a concrete derived class
{
public:
   virtual void reset();
   // ...
private:
   double _x, _y;
};

void Point::reset() { _x = _y = 0; }

int main() {
  Point p;
}