For each istreambuf_iterator constructor in this section, an end-of-stream iterator is constructed if and only if the exposition-only member sbuf_ is initialized with a null pointer value.
Effects: Initializes sbuf_ with nullptr.
istreambuf_iterator(istream_type& s) noexcept;
Effects: Initializes sbuf_ with s.rdbuf().
istreambuf_iterator(streambuf_type* s) noexcept;
Effects: Initializes sbuf_ with s.
istreambuf_iterator(const proxy& p) noexcept;
Effects: Initializes sbuf_ with p.sbuf_.