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From: barmar@think.com (Barry Margolin)
Newsgroups: comp.lang.eiffel,comp.object
Subject: Re: Reference Semantics
Message-ID: <1991Jun27.070321.23955@Think.COM>
Date: 27 Jun 91 07:03:21 GMT
References: <1991Jun21.215959.13966@cis.ohio-state.edu> <1991Jun22.175434.21450@cunews.carleton.ca> <1991Jun26.204549.27784@cis.ohio-state.edu>
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In article <1991Jun26.204549.27784@cis.ohio-state.edu> ogden@seal.cis.ohio-state.edu (William F Ogden) writes:
>Objects in the real world certainly have state. Actual examples of reference
>semantics, however, seem to be hard to come up with. Objects are visible
>and accessible to multiple entities in a physical environment, just as
>variables are in a programming environment, but that's clearly not analogous
>to having multiple references.

Why isn't it?

To continue the TV analogy, consider a class TV-WATCHER (representing
people who watch TV), with a slot TV-BEING-WATCHED, and a class
TELEVISION-SET.  If several TV-WATCHERs are in the same room, they would
all be watching the same TV, so they would have references to the same
TELEVISION-SET in their TV-BEING-WATCHED slots.

Sharing this reference is necessary to deal with state changes in the
TELEVISION-SET.  For instance, if one TV-WATCHER changes the channel, all
the TV-WATCHERs are now watching a different channel.  If each TV-WATCHER
had a copy of the TELEVISION-SET object, propagating these state changes
would be harder (you'd have to maintain a list of all the equivalent
TELEVISION-SETs).

-- 
Barry Margolin, Thinking Machines Corp.

barmar@think.com
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