CPP11 — Is SystemException supposed to be concrete?

Summary: The CORBA 2.2 C++ binding has the SystemException class defined as a
concrete class, because it redefines _raise() as a non-pure virtual.

This seems to be a bad idea to me. It would be better to leave _raise()
as a pure virtual so that SystemException cannot be instantiated. This
prevents accidental programming errors in catching SystemExceptions by
value, which slices off the real exception type.

Add pure virtual _raise functions to CORBA::SystemException and CORBA::UserException.