usage If X is of type B, we can use function F on X by writing X Fo only when f is a public member of either b or a 山东大学计算机科学与技术学院数据结构第5章堆栈山东大学计算机科学与技术学院 数据结构 第5章 堆栈 usage ◼ If X is of type B, we can use function F on X by writing X.F () only when F is a public member of either B or A. 8