c++ 排序，<< 运算符重载

#include <iostream>
#include <functional>
#include <list>
using namespace std;
ostream& operator<<(ostream& ostr, const list<int>& list)
{
    for (auto &i : list) {
        ostr << "-" << i;
    }
    return ostr;
}
int main()
{
    list<int> list = { ,,,,,,,,, };

    cout << "before:     " << list << "\n";
    list.sort();//升序
    cout << "ascending:  " << list << "\n";
    list.sort(greater<int>());//降序
    cout << "descending: " << list << "\n";
}

输出

before:     ----------
ascending:  ----------
descending: ----------