定义点类，然后由点类生成圆类，再由圆类生成圆柱体类。圆类的属性有圆心、半径；成员函数有构造函数、析构函数、显示、计算面积。圆柱体类的新增属性有高度；方法有构造函数、析构函数、显示、计算体积。类的应用：创建一个圆柱体对象，显示其信息，求它的体积。-Defined point category, then generates round classes　the point of class, then class is generated by the circular cylinder class. Property round class are center, radius　member functions have constructors, destructors, display, calculate the area. New property cylinder class of highly　methods are constructors, destructors, display, calculate the volume. Type of application: Create a cylinder object to display its information, find its volume.