对vector类进行多键值排序的方法.通过重载对指针的比较函数,实现了可以按照不同键值排序.-Category of vector multi-key sorting method. Through the overloading of function pointer comparison, realize that can be sorted according to different keys.

将存放在Vector对象中的200个Point对象排序，排序的条件为：按照X值从小到大排列-200 Point object sorting, will be stored in a Vector object sort conditions: in ascending order according to the value of X