想问为什么cmp函数比较order时不能有if？有就错没有就对？下面是不正确的代码，删去order那块的if条件判断语句，直接return就对，不理解

bool cmp(student s1, student s2)
{
    if (s1.year != s2.year) return s1.year < s2.year;
    if (s1.month != s2.month) return s1.month < s2.month;
    if (s1.day != s2.day) return s1.day < s2.day;
    if (s1.order != s2.order) return s1.order > s2.order;
}