Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!ucbvax!bloom-beacon!ai-lab!idsardi
From: idsardi@wheaties.ai.mit.edu (Bill Idsardi)
Newsgroups: comp.lang.c++
Subject: subclasses with missing members
Message-ID: <2484@wheat-chex.ai.mit.edu>
Date: 18 May 89 01:52:47 GMT
Reply-To: idsardi@wheat-chex.ai.mit.edu (Bill Idsardi)
Organization: The MIT AI Lab, Cambridge, MA
Lines: 37
References:

In reading a book recommended in this newsgroup recently
(Ira Pohl: C++ for C Programmers), there is a discussion
of a class, student, and an inheriting subclass, grad_student,
(pp 147ff):

	enum year {fresh, soph, junior, senior};
	enum support {ta, ra, fellowship, other};
	class student {
	protected:
		int	student_id;
		float	gpa;
	public:
	...
		year	yr;
	...
	}

	class grad_student : public student {
		support	s;
	public:
	...
	}

My question is:  Is there a (simple) way to cause grad_student
to fail to inherit the data member yr from student?

Another example of this would be a class of quadrilaterals
and a subclass of squares.  We might have 4 members representing
side lengths in quad, but only need 1 side length in square.
A similar point could be made about angles.

The obvious way is to invent an abstract class from which
student and grad_student would inherit.  Is there a better
way?

Bill Idsardi
MIT