istreambuf_iterator::istreambuf_iterator

istreambuf_iterator(streambuf_type *sb = 0) throw();
istreambuf_iterator(istream_type& is) throw();

The first constructor initializes the input stream-buffer pointer with sb. The second constructor initializes the input stream-buffer pointer with is.rdbuf(), then (eventually) attempts to extract and store an object of type E.