[filebuf.assign]

basic_filebuf& operator=(basic_filebuf&& rhs);

Effects: Calls close() then move assigns from rhs.

After the move assignment *this has the observable state it would have had if it had been move constructed from rhs (see [filebuf.cons]).

Returns: *this.

void swap(basic_filebuf& rhs);

Effects: Exchanges the state of *this and rhs.

template<class charT, class traits>
  void swap(basic_filebuf<charT, traits>& x, basic_filebuf<charT, traits>& y);

Effects: Equivalent to x.swap(y).

31 Input/output library [input.output]