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From: "Robert I. Eachus" <eachus@mitre.org>
Subject: Re: Freezing question
Date: 1999/07/19
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Ted Dennison wrote:
 
> > > Ahhh. I bet that's it. Does that mean that in the example above, Apple
> > > will inherit *none* of Fruit's operations, since none of them were
> > > declared before Apple?
> >
> > Correct.  See 7.3.1(7).

   (Well, actually it will inherit any implicit operations created by
the declaration.  In this case it gets "=".)

> Ahhh. OK. So then next question:
> How would one implement a hierarchy where one object needs dispatching
> operations that take a specific one of its chilren types as parameters?
> I'm running out of ideas.

    The usual is either to declare a class wide operation, and dispatch
in the body or to define the operation over a wider scope than necessary
and raise an exception when necessary.
 
> eg:
> 
>    type Widget is abstract tagged private;
>    type Container is abstract new Widget with private;
> 
>    procedure Add
>       (Child  : in out Widget;
>        Parent : in out Container'Class
>       );
> 
> Won't quite work because Add won't be inherited by any Containers unless
> it is placed before the declaration of Container. But if I move it
> between the two declarations, it won't compile because Container won't
> have been declared yet.

    Semantically, having the class of the parent derived from the child
doesn't make sense in all cases anyway, so I would do the following:

    type Widget is abstract tagged private;
    procedure Add (Child  : in out Widget;
                   Parent : in out Widget'Class);
    type Container is abstract new Widget with private;

    ...
    procedure Add (Child  : in out Widget
                   Parent : in out Widget'Class) is
    begin
      if not Parent in Container'Class then raise ... end if;

    Of course, IMHO, you want Widget and Container to be related so that
Parent is actually a wider class than Widget, but that is another
issue...
-- 

                                        Robert I. Eachus

with Standard_Disclaimer;
use  Standard_Disclaimer;
function Message (Text: in Clever_Ideas) return Better_Ideas is...