basic_stringbuf(ios_base::openmode mode =
ios_base::in | ios_base::out);
basic_stringbuf(basic_string<E, T, A>& x,
ios_base::openmode mode = ios_base::in | ios_base::out);The first constructor stores a null pointer in all the pointers controlling the input buffer and the output buffer. It also
stores mode as the stringbuf mode.
The second constructor allocates a copy of the sequence controlled by x, an object of class basic_string<E, T,
A>. If mode & ios_base::in is nonzero, it sets the input buffer to begin reading at the start of the sequence. If mode
& ios_base::out is nonzero, it sets the output buffer to begin writing at the start of the sequence. It also stores mode
as the stringbuf mode.