void swap(basic_ifstream& rhs);
Effects: Exchanges the state of 
*this
and 
rhs by calling
basic_istream<charT, traits>::swap(rhs) and
sb.swap(rhs.sb). template<class charT, class traits>
  void swap(basic_ifstream<charT, traits>& x, basic_ifstream<charT, traits>& y);
Effects: Equivalent to 
x.swap(y).