[istringstream.swap]
31 Input/output library [input.output]
31.8 String-based streams [string.streams]
31.8.3 Class template basic_istringstream [istringstream]
31.8.3.3 Swap [istringstream.swap]
void swap(basic_istringstream& rhs);
template<class charT, class traits, class Allocator>
void swap(basic_istringstream<charT, traits, Allocator>& x,
basic_istringstream<charT, traits, Allocator>& y);