[spanbuf.members]

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

Returns: If ios_base::out is set in mode, returns std::span<charT>(pbase(), pptr(), otherwise returns buf.

[Note 1:

In contrast to basic_stringbuf, the underlying sequence never grows and is not owned.

An owning copy can be obtained by converting the result to basic_string<charT>.

— end note]

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

Postconditions:

(3.1)
If ios_base::out is set in mode, pbase() == s.data() && epptr() == pbase() + s.size() is true;
- (3.1.1)
  in addition, if ios_base::ate is set in mode, pptr() == pbase() + s.size() is true,
- (3.1.2)
  otherwise pptr() == pbase() is true.
(3.2)
If ios_base::in is set in mode, eback() == s.data() && gptr() == eback() && egptr() == eback() + s.size() is true.

31 Input/output library [input.output]