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From: Florian Weimer <Florian.Weimer@RUS.Uni-Stuttgart.DE>
Newsgroups: comp.lang.ada
Subject: Re: A generic list package
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Date: 03 Sep 2001 18:18:35 +0200
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Date: 2001-09-03T18:18:35+02:00
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Stephen Leake <stephen.a.leake.1@gsfc.nasa.gov> writes:

>> I'm currently working on a general-purpose low-level list package.
>> The existing approaches were either too heavy-weight, too limited, or
>> covered by an unacceptable license.
>>
>> What do you think about the approach documented below?
>
> I've written a list package with similar goals; see
> http://users.erols.com/leakstan/Stephe/Ada/sal.html.

Thanks.  Your collection looks quite nice.

> I find it curious that you expect the client to provide the "next"
> field/operation, rather than declaring a Node_Type containing the
> user's type and a Next field. Can you elaborate on this?

Using this approach, you can save one indirection in the case of
indefinite types.  (The redirection seems necessary as well if you
need a non-constant view of the actual data.)  In such cases, your
approach seems to result in the following:

                                    +-------------- ...
       +------------------+         |
       |         o========#=======>>| actual data
       |                  |         |
       | fields specific  |         +-------------- ...
       | to the container |
       +------------------+


The getter/setter aproach permits the following:

       +------------------+
       | fields specific  |
       | to the container |
       |                  |
       |    actual data   |
       .                  .
       :                  :


And it is still possible to obtain a non-constant view of the actual
data.

> Your approach leaves everything up to the user; mine ensures that
> garbage is collected appropriately, when necessary; the user only
> needs to provide the appropriate 'free' operation.

I think our approaches are not much different in this regard; if an
element is removed from a list, it is also freed by my implementation.