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From: "Ehud Lamm" <mslamm@mscc.huji.ac.il>
Newsgroups: comp.lang.ada
Subject: Re: When to use 'Class in a parameter list
Date: Tue, 24 Jul 2001 18:29:08 +0300
Organization: The Hebrew University of Jerusalem
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Ed Falis <efalis@mediaone.net> wrote in message
news:3B5D8819.E9D8AD4E@mediaone.net...
> There are two situations I find useful for using classwide parameters.
>
> The first is when I want to force that the implementation of a routine
> will not be overridden by derived type.

Indedd. I failed to mention this reason, since it seemed like this is what
Marin focused on, where as I find the second reason of greater design
importance. But it is useful to keep this in mind (as well as trick for
making routines non-primitive, like declaring them in a child unit).

>
> The second is for implementing a "template pattern", where some overall
> processing is applied regardless of the specific type of the parameter,
> and dispatching is used within the routine to handle variation among
> specific types.  For instance, when traversing a tree, certain steps are
> applied to all nodes, whether internal or leaf, while others depend on
> the status of the node (composite pattern).  So here the traversal
> operation would use a classwide parameter, while the action applied
> within it, would dispatch.
>

Another example is having a "traversable" interface with
get_first/get_next/end_of_sequence operations, and creating various general
algorithms (like filters).

Ehud Lamm