Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!rutgers!bellcore!faline!thumper!ulysses!andante!alice!bs
From: bs@alice.UUCP (Bjarne Stroustrup)
Newsgroups: comp.lang.c++
Subject: Re: Problem with multiple inheritance? (Long)
Summary: virtual functions!
Message-ID: <8442@alice.UUCP>
Date: 19 Nov 88 02:49:04 GMT
References: <8335@nlm-mcs.arpa> <3438@jpl-devvax.JPL.NASA.GOV> <941@etive.ed.ac.uk>
Organization: AT&T Bell Laboratories, Murray Hill NJ
Lines: 25



Dave Berry, Laboratory for Foundations of Computer Science, Edinburgh.
beowulf.JPL.NASA.GOV (David Smyth):

 > >And more: you lose substitutablity when you use a virtual base class.
 > >Fancy cannot be on a list of windows, because you can't cast a pointer
 > >declared as a pointer to Window into a pointer to Fancy, or Bordered
 > >or Labeled for that matter.
 > 
 > Now this is a problem.  Can't C++ introduce a notion of cast that is
 > more sophisticated than the C notion, to handle cases like this?


Try virtual functions. Naturally, I could make casts work from a virtual
base class to a derived class (where the derived class is unambiguously
specified), but I don't see much point in such an elaboration since we
already have virtual functions as a cleaner mechanism.

 > If multiple inheritance doesn't provide the possibility of lists of
 > a given type including objects of a sub-type of that type, with
 > virtual members working properly, I don't think multiple inheritance
 > adds much, if anything, to the language.

Exactly, so it does.