constexpr ostreambuf_iterator() noexcept;
Effects: Initializes sbuf_ with nullptr.
ostreambuf_iterator(ostream_type& s) noexcept;
Requires: s.rdbuf() != nullptr.
Effects: Initializes sbuf_ with s.rdbuf().
ostreambuf_iterator(streambuf_type* s) noexcept;
Requires: s != nullptr.
Effects: Initializes sbuf_ with s.