basic_stringbuf& operator=(basic_stringbuf&& rhs);
Effects: After the move assignment *this has the observable state it would
have had if it had been move constructed from rhs (see [stringbuf.cons]).
void swap(basic_stringbuf& rhs);
Effects: Exchanges the state of *this
template <class charT, class traits, class Allocator>
void swap(basic_stringbuf<charT, traits, Allocator>& x,
basic_stringbuf<charT, traits, Allocator>& y);