虚函数（单一继承）

class Point {
public:
    virtual ~Point();
    
    virtual Point & mult (float) = 0;
    //.....
    float x() const { return _x;}
    virtual float y() const {return 0;}
    virtual float z() const {return 0;}
    
protected:
    Point( float x=0.0);
    float _x;
};

class Point2d : public Point{
public:
    Point2d (float x=0.0, float y=0.0) : Point(x), _y(y){}
    
    ~Point2d();
    
    Point2d & mult (float);
    float y() const {return _y;}
protected:
    float _y;
};

class Point3d : public Point2d{
public :
    Point3d (float x=0.0,float y=0.0, float z=0.0): Point2d(x,y), _z(z){}
    ~Point3d();
    
    Point3d & mult(float);
    float z() const { return _z;}
protected:
    float _z;
};

 --未完，待续