C++ 根据vector 容器内存放的自定义结构的某个元素 进行升序降序排序

a.h文件vector<Point2f> up4points；static bool  LessSortDown(Point2f &a, Point2f &b);//升序static bool  LessSortUp(Point2f &a, Point2f &b);//降序    a.cpp std::sort(up4points.begin(), up4points.end(), LessSortDown);bool  a::LessSortDown(Point2f &a, Point2f &b)//降序{return (a.x > b.x);}bool  a::LessSortUp(Point2f &a, Point2f &b)//升序{return (a.x < b.x);}