Manipulates the buffer.
Declared in <bslstl_stringbuf.h>
virtual
basic_streambuf<char_type, _Traits>*
setbuf(
char_type*,
streamsize);
Each derived class provides its own appropriate behavior. See the next-to-last paragraph of https://gcc.gnu.org/onlinedocs/libstdc++/manual/streambufs.html#io.streambuf.buffering for more on this function.
Base class version does nothing, returns this.