istream_iterator is an input iterator that reads successive elements from an input stream
istream_iterators require a template parameter this parameter is used to select the appropriate operator>> from the istream they are constructed with.
The istream_iterator::operator++ calls operator>>. Because the extraction is performed when the operator++ is used, the construction of an istream_iterator extracts a seed value from the istream so that it can immediately be read with the istream_iterator::operator*.
The default constructed istream_iterator is considered the "end-of-stream" iterator. Any istream_iterator of the same type which has reached the end of the istream it was constructed with will evaluate equal to the default constructed istream_iterator.
For more information: http://en.cppreference.com/w/cpp/iterator/istream_iterator
