Re: Downcasting - whats the problem?

From: Object Developer (ukobjectdeveloper_at_somewhere.co.uk)
Date: 05/18/04 

Date: Tue, 18 May 2004 20:41:18 +0000 (UTC)

"H. S. Lahman" <h.lahman@verizon.net> wrote in message
news:rgcqc.49214$L8.16342@nwrdny02.gnilink.net...
> Responding to Developer...
>
> > I would like to ask what will be a very basic question for many on
> > this list, so I hope those people offer some helpful advice on this.
> > I would like to understand what the real problem is with downcasting
> > in a staticly typed language. I know about the OCP, and I understand
> > the reasons for why this type of thing is wrong:
>
> The basic problem with downcasting is that it is treating an
> non-navigable relation as if it were a navigable association. An OO
> is-a relation is not navigable because there is only one instance in
> hand that resolves the entire tree. IOW, there is no to/from to navigate.

I wish I understand a word of that paragraph.

>
> In addition, accessing subclass specialization through the superclass is
> a no-no that defeats the point of restricting access to properties.
> When one accesses a superclass one is only supposed to be able to access
> the properties that the superclass makes available. If one needs to
> access subclass specializations, there should exist a relationship
> directly to the subclass that can be navigated for access to those
> properties.
>
>
> 1 R1 1
> [Client] ---------------- [Super]
> + doCommon()
> A
> | R2
> +----------+----------+
> | |
> [SubA] [SubB]
> + doA()
>
> [SubA] and [SubB] must provide implementations for doCommon(). If the
> solution requires Client to access doCommon() behavior all is well
> because that responsibility is exposed in [Super]. That is, Client
> navigates R1 -> Super to access the behavior.
>
> However, if a Client must access the doA() responsibility, it can't do
> that by "navigating" R1 -> Super -> R2 -> SubA to get to SubA. To
> access that specialization it needs a path directly to [SubA]:
>
>
> 1 R1 1
> [Client] ---------------- [Super]
> | 1 + doCommon()
> | A
> | | R2
> | R3 +----------+----------+
> | 1 | |
> +----------- [SubA] [SubB]
> + doA()
>
> so that it can navigate R3 -> SubA.
>

What happens with the OCP rule with this, with regards to the client, or
does it not matter?

> *************
> There is nothing wrong with me that could
> not be cured by a capful of Drano.
>
> H. S. Lahman
> hsl@pathfindermda.com
> Pathfinder Solutions -- Put MDA to Work
> http://www.pathfindermda.com
> (888)-OOA-PATH
>
>
>
>

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