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