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From: jhd@herold.franken.de (Joachim Durchholz)
Subject: Re: Real OO
Date: 1996/05/07
Message-ID: <68P8IzIk3RB@herold.franken.de>#1/1
X-Deja-AN: 153799894
distribution: world
references: <4j9p3a$uo5@watnews1.watson.ibm.com>
newsgroups: comp.lang.eiffel,comp.lang.ada,comp.object
Date: 1996-05-07T00:00:00+00:00
List-Id: <comp.lang.ada>


ncohen@watson.ibm.com wrote 06.05.96 on Re: Real OO:

> |> As I understand it, you mean the multiple dispatching mechanisms
> |> (a.k.a. classwide operations).
>
> No, that's not what classwide operatins are.

Ah, now some dark spots lighten up!

> (By "discriminated union," we mean that for each value V of each type in
> the class, there is a corresponding value in T'Class, and that the
> corresponding value in T'Class includes a "tag" identifying the type of
> the original value V.)

In Eiffel, each value from a class is also considered a value of its  
ancestor classes. The language definition does not offer a  
discriminating tag. (However, some libraries have a feature that will  
return the dynamic type of an object at run-time; this could be used  
as a tag if needed.)

> If you write a procedure
>
>    procedure Q(X: in A'Class);
>
> it is not an operation of type A, so it is not inherited in the usual
> sense by types derived from A.  However, in a call, the actual parameter
> may belong to A'Class, A, or any type descended from A.  Unlike P, Q
> never dispatches.

In Eiffel you'd declare Q "frozen". This means you're not allowed to  
override it, effectively making it non-dispatching.

> The body of Q must be written
> in such a way that it depends only on properties common to all types in
> the class.

Same in Eiffel - when writing Q, no features from descendant classes  
are yet available.

> THERE IS NO CLOS-LIKE
> MULTIPLE DISPATCH IN ADA.

Glad to hear that. Sorry for bashing the wrong language.

>  You CAN declare
>
>    procedure S(X: in A'Class; Y: in B'Class);
>
> and this poses no problems.  It does not dispatch.

Well, Eiffel doesn't allow you to freeze parameter classes, so we have  
a point where Ada allows a bit more freedom. I can't comment on this  
yet - I'll have to revisit everything in this thread that had the  
words "class-wide" in it, which will take me some time...

-Joachim

--
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