建立平面上一个的点类，该类内包含求该点到另一个点的距离的成员 函数。根据该类建立一个派生类，派生类表示空间内的一个点，并求 该点到空间内另一点的距离。 -The establishment of a point on the plane class contains requirements that point to another member of the distance function. According to such a derived class, derived