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From: billwolf@hubcap.clemson.edu (William Thomas Wolfe,2847,)
Newsgroups: comp.lang.ada
Subject: Re: Garbage Collection
Message-ID: <3861@hubcap.UUCP>
Date: 13 Dec 88 18:37:34 GMT
References: <6702@june.cs.washington.edu>
Sender: news@hubcap.UUCP
Reply-To: wtwolfe@hubcap.clemson.edu
List-Id: <comp.lang.ada>

>From article <6702@june.cs.washington.edu>, by klaiber@june.cs.washington.edu (Alexander Klaiber):
> Assume I maintain mailing-lists of people and I represent the lists by
> pointers to objects representing one person each; i.e.
> 
> type person is (some adt)
> type mailinglist is new list(person);
> 
> Now if I have more than one mailing-list, obviously I have structure-sharing: 
> a given person may appear on more than one list. 

    No, this isn't structural sharing, because you are explicitly manipulating 
    a list of pointers.  Structural sharing occurs when you have two objects, 
    A and B, and making some modification to B causes a modification to A as 
    well, because they share a portion of their structure.  The structure
    of a pointer is simply the address field, and the address fields are not
    being shared.  Now if we had two ADTs, both implemented by *hidden*
    pointers, and assignment failed to result in a deep copy, then THAT 
    would be structural sharing. 

    In this particular instance, it would be best to store the key by which
    a person could be identified (social security number, for example) in
    the list, and then using the key to retrieve the current address from
    the Person database.  Thus, a mailing list would be a list of social
    security numbers. 
   

                                            Bill Wolfe

                                    wtwolfe@hubcap.clemson.edu