Re: Dynamic redispatching

[Ray Blaak <blaak@infomatch.com>]

Ted Dennison <dennison@telepath.com> writes:
> I'm trying to write a classwide function that dynamicly redispatches
> based on the tag of the function result. But it isn't dispatching on
> either of the two Ada compilers I tried. Here's an example:
[...]
>    function Main_Dispatcher return Class is
>    begin
>       return Dispatch;
>    end Main_Dispatcher;
[...]
> procedure Test is
>    Item : P1.Handle := new P2.Instance;
> begin
>    Item.all := P1.Main_Dispatcher;
> end Test;
> 
> When I run Test, of course I want it to dispatch to P2.Dispatch and
> return a value of type P2.Instance.

The problem is that you need an object to dispatch on. There is no information
inside Main_Dispatcher to decide which Dispatch to call, since by the time the
object is returned, it is too late (or more accurately, you are specifying only
to call P1.Dispatch).

In the statement "Item.all := P1.Main_Dispatcher", the contents of Item are
irrelevant, since only Main_Dispatcher can be called. Inside Main_Dispatcher,
no information about the result destination is known.

Perhaps this would work:

   Item.all := P1.Dispatch;

For then we have a function with a controlling result (I think), and it should
thus use the context (the tag of the destination) to dispatch.

I think the main problem was that Main_Dispatcher is not a primitive operation
of P1.Instance

-- 
Cheers,                                        The Rhythm is around me,
                                               The Rhythm has control.
Ray Blaak                                      The Rhythm is inside me,
blaak@infomatch.com                            The Rhythm has my soul.