多态继承的一个小例子，mark一下。

#include <iostream>
using namespace std;

class Point2d{
public:
	Point2d( float x = 0.0, float y = 0.0) : _x(x),_y(y){};
	float x(){ return _x; }
	float y(){ return _y; }
	virtual float z(){ return 0.0;}
	virtual void z(float){ }
	virtual void operator+=(Point2d& rhs)
	{
		_x += rhs.x();
		_y += rhs.y();
	}
protected:
	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){};
	float z(){ return _z;}
	void z(float newZ){ _z=newZ; }
	void operator+=(Point2d& rhs)
	{
		Point2d::operator+=(rhs);
		_z += rhs.z();
	}
protected:
	float _z;
};

int main()
{
	Point2d p2d(2.1, 2.2);
	Point3d p3d(3.1, 3.2, 3.3);
	p3d += p2d;
	cout<<"p3d data members: "<<p3d.x()<<" "<<p3d.y()<<" "<<p3d.z()<<endl;
}