basic_ostream& operator<<(basic_ostream& (*pf)(basic_ostream&));
basic_ostream& operator<<(basic_ios<charT, traits>& (*pf)(basic_ios<charT, traits>&));
basic_ostream& operator<<(ios_base& (*pf)(ios_base&));
basic_ostream& operator<<(basic_streambuf<charT, traits>* sb);
basic_ostream& operator<<(nullptr_t);