constexpr istreambuf_iterator() noexcept;
constexpr istreambuf_iterator(default_sentinel) noexcept;
Effects: Constructs the end-of-stream iterator.
istreambuf_iterator(basic_istream<charT, traits>& s) noexcept;
istreambuf_iterator(basic_streambuf<charT, traits>* s) noexcept;
Effects: Constructs an istreambuf_iterator that uses the basic_streambuf object *(s.rdbuf()), or *s, respectively. Constructs an end-of-stream iterator if s.rdbuf() is null.
istreambuf_iterator(const proxy& p) noexcept;
Effects: Constructs a istreambuf_iterator that uses the basic_streambuf object pointed to by the proxy object's constructor argument p.