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From: ogden@seal.cis.ohio-state.edu (William F Ogden)
Subject: Re: Reference Semantics
Message-ID: <1991Jun26.204549.27784@cis.ohio-state.edu>
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Organization: The Ohio State University, Department of Computer and Information Science
References: <1991Jun19.001829.28317@syacus.acus.oz.au> <1991Jun21.215959.13966@cis.ohio-state.edu> <1991Jun22.175434.21450@cunews.carleton.ca>
Date: Wed, 26 Jun 1991 20:45:49 GMT
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In article <1991Jun22.175434.21450@cunews.carleton.ca> knight@mrco.carleton.ca (Alan Knight) writes:

>>>As object oriented programming reflects the real world, it seems that
>>>references will be inevitable.
>>At the risk of getting back into those dangerous analogies, note that
>>referencs semantics really entail multiple larger objects routinely sharing
>>possesion of smaller objects. For very good reasons, this rarely happens
>>in the real world. Imagine, for example, several people in a room each
>>possesing a remote tuner for the room's only television set. The chaos
   ...
>  The phrase "larger objects ... sharing possession of smaller
>objects" implies a physical part-of relationship, sharing of which
>doesn't often happen in the real world.   That doesn't mean that
>objects aren't shared.

The issue here certainly wasn't that objects aren't and shouldn't be
shared -- both in the programming and in the real world. The concern was
that multireferences to objects entails the simultaneous _shared possession_
of those objects by other entities in the system. 

>  In fact, in the "real" world that I live in, objects are usually
>shared, and copying is fairly rare.  At my house, we have one
>television set which is shared between all the residents.  It's true
>that there's only one remote control, but if my wife decides to change
>the state of the television set, it certainly affects the copy that
>I'm referencing.

In your world then there is only one television object whose use is shared
by several other entities. The entities are aware of each other and that in
order to function properly, they must set up and obey certain conventions
regarding the use of the television. [Supposing that a remote controller
is an adequate analogue of a reference to a TV object,] you then follow
the simplifying convention of only having ONE reference to your TV and of
sharing (at different times) the use of that reference. If you had several
TV's and remote tuners which could selectively control any TV, then the
physical situation here might begin to approach the complexity inherent
in reference semantics.

>  Copying semantics (in which as soon as I walk into the room I get my
>own television to look at) would be rather disconcerting.
>Copy-on-write would be rather entertaining, if anyone who changed the
>channel got a new television set. ...

As I said early in this discussion, copying of large objects is
difficult and expensive, and it should be avoided if at all possible.

>  To actually make a point:  The real world has state and reference
>semantics all over the place.  There may be good arguments for
>avoiding them, but the claim that they are not used in the real world
>is not one of them.

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.