Deriving_a_user_class
 If you derive your own class from the ios class, or from one of
 its derived classes, the ios sub-object must be properly initialized
 during instantiation.  Specifically, you must ensure that the
 streambuf pointer within the ios sub-object is valid. To do this, you
 can specify the ios(streambuf *) constructor as a member initializer
 for your class constructor.  Optionally, you can call the
 ios::init(streambuf *) member function.