c++ list sort方法

#include <list>
#include <string>
#include <functional>
#include <iostream>

struct S {
         std::string firstname;
         std::string secondname;
         int ID;
         // 重新定義小於，因為預設的sort函式呼叫的操作符是<，所以我們只需要過載 < 就好了
        bool operator < (S & b) {
            return ID < b.ID;
         }

};
int main(int argc, char* argv[])
{
           std::list<S> mylist;
           std::list<S>::iterator iter;
           S a;
           a.firstname ="dfadf";
           a.ID = 5;
           mylist.push_back (a);
          a.firstname ="得到";
           a.ID = 9;
           mylist.push_back (a);
           a.firstname ="xxx";
           a.ID = 7;
          mylist.push_back (a);
          a.firstname ="gggg";
          a.ID = 25;
           mylist.push_back (a);
            mylist.sort();
          // 現在預設的operator已經被我們過載了，就不用管，直接呼叫sort就好了
           for (iter = mylist.begin(); iter != mylist.end();++iter)
           {
                     std::cout <<static_cast<S>(*iter).ID << "\t";
            }
           std::cout <<std::endl;
           return 0;
 }