Re: Overloading for T and T'Class

[dweller@Starbase.NeoSoft.COM (David Weller)]

In article <CzC6Ho.Lqn@swlvx2.msd.ray.com>,
John Volan <jgv@swl.msd.ray.com> wrote:
>Thanks to those who sent me private e-mail on this.  Summarizing the
>responses so far: Yes, this does appear to be legal according to the
>current definition of Ada9X.  Apparently, T and T'Class are considered
>to be different types, even though the set of values of type T'Class
>"covers" the set of values of type T.  That's pretty much what I
>figured.
>
>The other half of my question was whether there was any rationale
>anyone could suggest for actually making use of this kind of
>overloading.  Personally, I haven't been able to come up with one.  On
>the basis of the ambiguous-invocation problem alone, I'd say that we'd
>probably want to avoid doing overloadings like these.  But does anyone
>else have any thoughts on this?
>

(Dave rubs hands gleefully)

Why, yes, John, there _is_ a use.  The Booch Components has the
ability to permit assignment and equality checks across each
hierarchy.  THat way, I can see if an Unbounded_Queue is structurally
equivalent to a Bounded_Queue.  Sooooo....  I create a top level type
declaration that is abstract:
	type Queue is abstract tagged private;
then an "=" operator:
	function "="(L, R : Queue'Class) return Boolean;
(There is an implicit declaration of:
	function "="(L, R : Queue) return Boolean;
when that classwide operation is declared)

Now, the point is that I can _never_ call the operation "=" for type
Queue (except perhaps inside the body of the package where Queue is
declared, and I can handle that).  

OK, that doesn't _quite_ answer your question, but it does suggest a
potential area for application.

I'd give a longer dissertation on this, but I'm still finishing up
slides for a presentation on the Booch Components for tonight.

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