istream_iterator, black magic?

Question

I don't understand why the second argument for_each is an empty iterator. Can someone explain this to me, please?

fstream("datfile.txt"); 
for_each(istream_iterator<string>(datfile), istream_iterator<string>(), rand_func);

Answer 1

这是一个特殊的“前哨”值，其唯一目的是与另一个达到EOF的istream_iterator()比较时返回值true 。

Answer 2

The default constructed istream_iterator is used as a sentinel that denotes the end of stream condition. When reading from the istream ends, the first iterator input to for_each will equal this sentinel iterator, thus terminating the for_each .