The istream_iterator is another very handy adapter. It can be used with any std::istream object (which can be the standard input or files for example) and will try to parse the input from that stream object according to the template parameter it was instantiated with. In this section, we used std::istream_iterator<int>(std::cin), which pulls integers out of the standard input for us.

The special thing about streams is that we often cannot know in advance how long the stream is. That leaves the question, where will the end iterator point to if we do not know where the stream's end is? The way this works is that the iterator knows when it reaches the end of the stream. When it is compared to the end iterator, it will effectively not really compare itself with the end iterator but return if the stream has any tokens left. That's why the end iterator constructor does not take any arguments.