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From: Adam Beneschan <adam@irvine.com>
Newsgroups: comp.lang.ada
Subject: Re: Generics with concrete and class-wide types
Date: Tue, 1 Apr 2008 14:17:56 -0700 (PDT)
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On Apr 1, 1:10 pm, "Randy Brukardt" <ra...@rrsoftware.com> wrote:
> "Adam Beneschan" <a...@irvine.com> wrote in message
>
> news:f0879dc0-7498-48f7-8d44-9856316d35ce@s19g2000prg.googlegroups.com...
> ...
>
> > What's missing here is a way to specify a generic formal subprogram
> > that must be a primitive operation of some tagged type (possibly a
> > generic formal tagged type),
>
> No, it's not missing. That's what abstract formal subprograms are for. See
> specifically 12.6(8.4-8.5/2).

Ah ha, thank you!  No, I didn't realize that's what they were for,
perhaps because "abstract" isn't a synonym for "dispatching", but it
makes sense now that you pointed me in the right direction.


> I'm not sure that helps in this case, but feel free to try. ;-).

I think it does help.  If my previous example were changed to this:

    generic
        type Element is private;
        type Iterator_Root is tagged private;
        with function Get (I : in Iterator_Root) return Element
           is abstract <>;  -- unspeakable syntax??
        ... other operations
    package SP_For_Class is
        procedure Some_Procedure_Class (I : in Iterator_Root'Class);
    end SP_For_Class;

    with Some_Procedure;
    package body SP_For_Class is
        function Dispatching_Get (I : in Iterator_Root'Class)
                    return Element is
        begin
            return Get (I);
        end Dispatching_Get;
        ... similarly for other operations
        procedure SP_Inst is new Some_Procedure
            (Element, Iterator_Root'Class,
             Dispatching_Get, ...other operations);
        procedure Some_Procedure_Class (I : in Iterator_Root'Class)
            renames SP_Inst;
    end SP_For_Class;

then the call to Get inside Dispatching_Get would be legal.  Then, in
Maciej's case, if he wants a Some_Procedure instance that would work
on a class-wide type and dispatch, he could instantiate SP_For_Class
with the specific type, and then Some_Procedure_Class declared in the
instance would be the procedure he's looking for.  At least I think
this would work---I haven't tried it.  It's a little klunky to declare
the SP_For_Class generic, but at least it wouldn't involve any
duplicated code.  And if it works, it will work without waiting for a
solution to AI05-71.

                               -- Adam