[ofstream.swap]
31 Input/output library [input.output]
31.10 File-based streams [file.streams]
31.10.5 Class template basic_ofstream [ofstream]
31.10.5.3 Swap [ofstream.swap]
void swap(basic_ofstream& rhs);
template<class charT, class traits>
void swap(basic_ofstream<charT, traits>& x, basic_ofstream<charT, traits>& y);