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 class represents a point in space, and find the space to another point to point distance.