说明：  该程序具有优良的扩展性能。如果想要添加新的绘图元素，那么只需要添加一个以CdrawElement类为基类的绘图类即可，从而非常方便对程序进行扩展。
(the procedure with excellent scalability. If you want to add a new graphics elements, then only need to add a category for CdrawElement base class graphics category can be, thus very convenient procedure for expansion.)