basic_istringstream::basic_istringstream

explicit basic_istringstream(ios_base::openmode mode = ios_base::in);
explicit basic_istringstream(const basic_string<E, T, A>& x,
    ios_base::openmode mode = ios_base::in);

The first constructor initializes the base class by calling basic_istream(sb), where sb is the stored object of class basic_stringbuf<E, T, A>. It also initializes sb by calling basic_stringbuf<E, T, A>(mode | ios_base::in).

The second constructor initializes the base class by calling basic_istream(sb). It also initializes sb by calling basic_stringbuf<E, T, A>(x, mode | ios_base::in).