我知道年份总是每行第54到58个字符

"I know that the year is always the 54th to 58th character in each line"

因此，请考虑：

int main ()
{
    ifstream  in("laugavegurinn.txt", ios::in);
    ofstream out("laugavegurinn2.txt");
    string str;
    multimap<int, string> datayear;

    while (getline(in,str)) {
         int year = stoi(str.substr(54, 4));
         datayear.insert(make_pair(year,str)); //use insert function
         // multimap don't have [] operator
           }

    for (auto v : datayear) 
        out << v.second << "\n";

    in.close();
    out.close();
}