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From: Stephen Leake <stephen.a.leake.1@gsfc.nasa.gov>
Newsgroups: comp.lang.ada
Subject: Re: List container strawman
Date: 13 Nov 2001 11:29:01 -0500
Organization: NASA Goddard Space Flight Center
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Simon Wright <simon@pushface.org> writes:

> Stephen Leake <stephen.a.leake.1@gsfc.nasa.gov> writes:
> 
> > "List" should be limited, to prevent non-deep copies when Element is
> > an access type.
> 
> I don't see why you would want to do this specifically? (perhaps I've
> misunderstood).
> 
> If I make a List of Bank_Account_Access whose 5th element is a pointer
> to my bank account, and then make a copy of it, what would you expect
> the 5th element of the new List to point to? Better not be a *copy* of
> my bank account!

Good example. But there are many other examples where copying a list
does _not_ make sense. For example, if I have a list of controls in a
GUI dialog box, and I pop up another dialog box, I want a list of
_new_ controls, not the same ones.

So the base list container must allow the user to control the copying
process. That means either making it Limited and providing a Copy
operation, or making it Controlled and providing Adjust. In either
case, the user of the package must provide a Copy_Element operation
that does the Right Thing.

-- 
-- Stephe