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From: bbalfour@std.caci.com (Brad Balfour)
Subject: Re: How to overload assignment in Ada 95?
Date: 1996/12/10
Message-ID: <bbalfour-1012961302000001@stmac0088.std.caci.com>#1/1
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distribution: usa
references: <58aclh$ssd@gaia.cc.gatech.edu>
 <bbalfour-0912961334030001@stmac0088.std.caci.com>
 <mheaney-ya023280000912962054110001@news.ni.net>
organization: CACI, Inc.
reply-to: bbalfour@std.caci.com
newsgroups: comp.lang.ada
Date: 1996-12-10T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <mheaney-ya023280000912962054110001@news.ni.net>,
mheaney@ni.net (Matthew Heaney) wrote:
[snip]
>Does anyone have a rule of thumb for when a controlled type should publicly
>derive from type Ada.Finalization.Controlled?
[snip]
>In the former case, clients of the Unbounded_Stack have direct visibility
>to the operations Initialize, Finalize, and Adjust.  When is this a good
>thing?  A bad thing?

This exact question was dealt with in the next two articles in the Tips &
Tidbits series (see <http://www.acm.org/~bbalfour/tips_and_tidbits.html>):

 Number 2: Expressing Design Inheritance Relationships in Ada 95,
   published in Ada Letters, Vol. XV No. 3, May/June, 1995. 
         <http://www.acm.org/~bbalfour/tips_no_2.html>
 Number 3: Inheritance and Child Library Units, published in Ada
      Letters, Vol. XV No. 4, July/August, 1995.
         <http://www.acm.org/~bbalfour/tips_no_3.html>

There are actually two separate issues/trade-offs:
1) A visible extension vs a private extension of the parent and
2) Derivation in the visible part vs. a private type and derivation
    in the private part.
The above two articles discuss when each technique is appropriate.

However, one conclusion has become fairly obvious in retrospect. If
overridings of Adjust, Finalize and Initialize are provided in the visible
part, then it is possible for clients to call them directly -- a situation
that is *not* desireable. Therefore, I now recommended that the overriding
declarations appear in the private part. However, moving the overridings
to the private part opens the (small) possiblity that the type declaration
becomes frozen prior to these declarations. If this were to happen, the
compiler would issue an error message. At that point, the developer can
then modify the code to move the declarations prior to the freezing point.

>--------------------------------------------------------------------
>Matthew Heaney
>Software Development Consultant
>mheaney@ni.net
>(818) 985-1271

-- 
Brad Balfour                            SIGAda WWW Server
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