template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, basic_string_view<charT, traits> str);

Forms a character sequence seq, initially consisting of the elements defined by the range [str.begin(), str.end()).

Then inserts seq as if by calling os.rdbuf()->sputn(​seq, n), where n is the larger of os.width() and str.size(); then calls os.​width(0).