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From: billwolf@hubcap.clemson.edu (William Thomas Wolfe,2847,)
Newsgroups: comp.lang.ada
Subject: Re: Garbage Collection
Message-ID: <3917@hubcap.UUCP>
Date: 15 Dec 88 19:13:41 GMT
References: <4159@enea.se>
Sender: news@hubcap.UUCP
Reply-To: billwolf@hubcap.clemson.edu
List-Id: <comp.lang.ada>

>From article <4159@enea.se>, by sommar@enea.se (Erland Sommarskog):
> But you have to write the same boring code for [destroying]
> every ADT you implement!

     If there were an ability to iterate over the fields of a
     generic record parameter, standard generics for recursively
     destroying certain classes of structures (e.g., n-ary trees)
     could be written, thus automating the process to some extent.  

     The reason we must suffer writing the same code repeatedly is that 
     Ada does not permit us to abstract over arbitrary record structures. 

> Let's say that you write a package for double-linked lists. You provide 
> a procedure for taking an element out of a list. Should you free the 
> memory allocated? You don't know because the caller may or may not still 
> keep an reference to the element. He may be totally uninterested in it, 
> but he may also want to insert it in another list.  

    When the list is instantiated, the user tells us what type of object
    is to be kept in the list, and provides us with procedures for
    assignment and equality.  Objects are copied into and out of the list.  
    When an object is to be removed from the list, the local copy of the
    object which is stored within the list is destroyed.  Now if the
    user wants, a pointer type can be supplied as the type of object
    to be listed, and the user-supplied equality function can compare
    the objects pointed to rather than the pointers themselves.  The
    user then bears all responsibility for managing the "external storage",
    and can track his/her own references.

    There is no implementor's dilemma here whatsoever.


                                          Bill Wolfe

                                    wtwolfe@hubcap.clemson.edu