Path: utzoo!censor!geac!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!lll-winken!unixhub!shelby!neon!craig
From: craig@Neon.Stanford.EDU (Craig D. Chambers)
Newsgroups: comp.object
Subject: Re: Examples of Multiple Inheritance?
Keywords: multiple inheritance
Message-ID: <1990Dec17.223306.25756@Neon.Stanford.EDU>
Date: 17 Dec 90 22:33:06 GMT
References: <18090@neptune.inf.ethz.ch> <1990Dec13.035418.28040@Neon.Stanford.EDU> <47353@apple.Apple.COM>
Organization: Stanford University
Lines: 30

In article <47353@apple.Apple.COM> lins@Apple.COM (Chuck Lins) writes:
>In article <1990Dec13.035418.28040@Neon.Stanford.EDU> craig@Neon.Stanford.EDU (Craig D. Chambers) writes:
>>If you're concerned about space overheads of MI, other implementations
>>of MI than the one in C++ 2.0+ have low space overheads, comparable to
>>pre-MI C++ implementations (i.e. a word or two extra per object).
>
>Could you provide some examples or references to papers describing such
>implementations? For some of us this (oo research) is a part-time endeavor
>and we don't always have the time to read every single paper. (Our limits,
>not yours.) Thanks in advance.

Languages based on separate hash tables for message lookup (like Self,
Smalltalk, many Lisp-based systems, Trellis/Owl, and (I think) Eiffel)
can use one extra word in the beginning of the object to identify the
object's run-time type.  This could either be a pointer to a class
object, an invisible map object as in Self, or a table of defined
methods.  No extra words per object are needed.  C++'s MI
implementation is designed to trade away some space per object to get
faster message lookup (although in-line caching may be faster than
this implementation when the cache hits, especially when combined with
customization).

I think the Smalltalk Blue Book [Golberg & Robson '83] might talk
about this representation towards the back, and the Self
implementation is summarized in OOPSLA'89.  As far as I know,
Trellis[/Owl]'s implementation has only been described in unpublished
documents.  I don't know if Eiffel's implementation is described
anywhere.

-- Craig Chambers