As the iostream class hierarchy diagram shows, the ios class is the base class for all the input/output stream classes. While ios is not technically an abstract base class, you will not usually construct ios objects, nor will you derive classes directly from ios. Instead, you will use the derived classes istream and ostream or other derived classes.
Even though you will not use ios directly, you will be using many of the inherited member functions and data members described here. Remember that these inherited member function descriptions are not duplicated for derived classes.
#include <iostream.h>
istream, ostream