std::span<CharT> span() const noexcept;

void span( std::span<CharT> s ) noexcept;

1) Gets a span referencing the written area if std::ios_base::out is set in the open mode, or a span referencing the underlying buffer otherwise.

2) Makes the basic_spanbuf perform I/O on the buffer referenced by s. Sets pointers to get area, put area, or both.

Set bits in open mode (affecting pointers to get area)	Return value after setting
	eback()	gptr()	egptr()
std::ios_base::in	s.data()	s.data()	s.data() + s.size()
Set bits in open mode (affecting pointers to put area)	Return value after setting
	pbase()	pptr()	epptr()
std::ios_base::out && !std::ios_base::ate	s.data()	s.data()	s.data() + s.size()
std::ios_base::out && std::ios_base::ate	s.data()	s.data() + s.size()	s.data() + s.size()

1) std::span<CharT>(pbase(), pptr()) if std::ios_base::out is set in the open mode, or a std::span<CharT> that references the whole underlying buffer otherwise.

2) (none)