一个类虚函数表，虚表指针到底有几个

class point2d {
public:
        point2d(float x = 0.0, float y = 0.0)
                : _x(x), _y(y) {};
   virtual ~point2d() {};

        virtual float x() { return _x; };
        virtual float y() { return _y; };
        virtual float z() { return 0.0; };
private:
        float _x, _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() {};
       
        float z() { return _z; };
private:
        float _z;
};