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From: Vinzent Hoefler <ada.rocks@jlfencey.com>
Newsgroups: comp.lang.ada
Subject: Re: Ada 200X
Date: Tue, 03 Jun 2003 11:09:50 +0200
Organization: JeLlyFish software
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Date: 2003-06-03T11:09:50+02:00
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Robert I. Eachus wrote:

>Vinzent Hoefler wrote:
>
>> Because the view conversion to a grandparent is of course allowed,
>> this is little tricky if not to say error prone sometime.
>>=20
>> Well, yes, I know, you should always design your object hierarchy
>> beforehand. ;-)
>
>In one sense this is a valid criticism of the "Ada way" of doing things.=
=20
>  In another it is totally bogus.

I was afraid so. ;-)

>Ada tagged type derivation trees tend to come in two flavors. [...]

Mmh, well. I encounter a third variant quite often. For instance let's
take a short look at a part of the good old TurboVision object
hierarchy:

|                         +-------+
|                         =A6TObject=A6
|                         +-------+
|                             |
|                          +-----+
|                          =A6TView=A6
|                          +-----+
|                             |
|       +----------------------------------------------+
|       |       |        |       |      |      |       |
|  +----------+ |  +----------+  | +--------+  |   +------+
|  |TScrollBar| |  |TInputLine|  | |TCluster|  |   |TGroup|
|  +----------+ |  +----------+  | +--------+  |   +------+
|               |                |     |       |       |
|         +-----------+     +--------+ | +-----------+ +-------------+
|         =A6TStaticText=A6     =A6THistory=A6 | =A6TListViewer=A6       =
        |
|         +-----------+     +--------+ | +-----------+               |
|               |                      |       |                     |
|         +-----------+                |    +-----------+            |
|         |           |                |    |           |            |
|      +------+  +----------+          | +--------+ +--------------+ |
|      |TLabel|  |TParamText|          | |TListBox| |THistoryViewer| |
|      +------+  +----------+          | +--------+ +--------------+ |
|                               +------+--------+           +--------+
|                               |      |        |           |
|                               | +-----------+ |       +-------+
|                               | |TCheckBoxes| |       |TWindow|
|                               | +-----------+ |       +-------+
|                         +-------------+ +----------------+ |
|                         |TRadioButtons| |TMultiCheckBoxes| |
|                         +-------------+ +----------------+ |
|                                                    +-------------+
|                                                    |             |
|                                            +--------------+  +-------+
|                                            |THistoryWindow|  |TDialog|
|                                            +--------------+  +-------+

This rather looks like a tree. And I doubt, any medium sized OO-System
would be much different from that just because it's Ada.

So let's say, as a specific example, I am deriving MyWindow from
TWindow, slightly change one of its method and let the fathers method
do the usual rest. Now I decide to insert another object (of course I
will derive from that later) into the tree, so that TWindow is now the
grandfather of MyWindow. Because I am still calling the method of
TWindow I skip the new inbetween object's method. This might be wrong
or false depending on what it does and what I intended to do. So
having something like "inherited" in Object-Pascal that automagically
selects the right method of either the father, grandfather,
grand-grandfather, ..., in the cases where I *really* just want to use
the method of the anchestor would be nice.

Well, after a night of thought I see some problems with that,
especially that it 1) hides where exactly the method comes from
(depending on the view of information hiding this might be a good or a
bad thing) and 2) a worse case: If the anchestor, whos method I am
about to call, is really declared in another package that I didn't
even "with"ed in.


Vinzent.

--=20
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