Path: utzoo!utgpu!utstat!jarvis.csri.toronto.edu!mailrus!ncar!boulder!sunybcs!sher
From: sher@sunybcs.uucp (David Sher)
Newsgroups: comp.lang.c++
Subject: Why no virtual data?
Message-ID: <4606@cs.Buffalo.EDU>
Date: 9 Mar 89 18:53:58 GMT
Sender: nobody@cs.Buffalo.EDU
Reply-To: sher@cs.Buffalo.EDU (David Sher)
Organization: SUNY/Buffalo Computer Science
Lines: 30

Is there any reason why functions can be virtual but not data objects
(the implementation of a virtual data object is exactly analogous to that
of a virtual function).  Here's the application:
I have a matrix package with two types of matrices and vectors.
A vector is the base class is what you'd expect.
A spaced vector is a vector which is not contiguous in memory but is
every nth element where n is given.
Similarly I have matrix and spaced matrix.  So a row of a matrix is
a vector.  Thus I can define
class matrix {...
 virtual vector *rows;  //  the rows of the matrix
 ...
};

I also want to say:
class spaced_matrix {...
 virtual spaced_vector *rows;
 ...
}

Currently c++ does not support this but I would think that this would
increase the orthogonality of the language, the efficiency of implementing
things like this (which would seem to be a fairly common effect the
elements of a complex subclass would be sub classes) and generally 
a trivial to implement improvement.  Adding it would not break anything
I can imagine and in fact should simplify the compiler considerably.
-David Sher
-David Sher
ARPA: sher@cs.buffalo.edu	BITNET: sher@sunybcs
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