虚函数（单一继承）

 1 class Point {
 2 public:
 3     virtual ~Point();
 4     
 5     virtual Point & mult (float) = 0;
 6     //.....
 7     float x() const { return _x;}
 8     virtual float y() const {return 0;}
 9     virtual float z() const {return 0;}
10     
11 protected:
12     Point( float x=0.0);
13     float _x;
14 };
15 
16 class Point2d : public Point{
17 public:
18     Point2d (float x=0.0, float y=0.0) : Point(x), _y(y){}
19     
20     ~Point2d();
21     
22     Point2d & mult (float);
23     float y() const {return _y;}
24 protected:
25     float _y;
26 };
27 
28 class Point3d : public Point2d{
29 public :
30     Point3d (float x=0.0,float y=0.0, float z=0.0): Point2d(x,y), _z(z){}
31     ~Point3d();
32     
33     Point3d & mult(float);
34     float z() const { return _z;}
35 protected:
36     float _z;
37 };
--未完，待续