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From: jsa@alexandria (Jon S Anthony)
Subject: Re: Invoking parental methods (was: Java vs Ada 95)
Date: 1996/11/08
Message-ID: <JSA.96Nov8141603@alexandria>#1/1
X-Deja-AN: 195344700
sender: news@organon.com (news)
references: <E0EIM2.A3p.0.-s@inmet.camb.inmet.com>
organization: Organon Motives, Inc.
newsgroups: comp.lang.ada
Date: 1996-11-08T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <E0GAKr.HB.0.-s@inmet.camb.inmet.com> mg@asp.camb.inmet.com (Mitch Gart) writes:

>     procedure supertest is
> 	package p1 is
> 	    type parent_obj is tagged null record;
> 	    type super is access all parent_obj'class;
> 	    procedure p_first (param: access parent_obj);
> 	end p1;
...[snip]...
> 	package p2 is
> 	    type child_obj is new p1.parent_obj with null record;
> 	    procedure p_first (param: access child_obj);
> 	end p2;
> 	package body p2 is
> 	    procedure p_first (param: access child_obj) is
> 	    begin
> 		put_line("child p_first");
> 		p1.p_first(p1.super(param));	-- ***
> 	    end; 
> 	end p2;

> The line marked *** dispatches back to p2.p_first, causing an
> infinite recursion.  To get this one to do what's wanted, the

I'm not sure why anyone would think otherwise.  I mean you haven't changed
anything about the thing referenced.  Would anyone be confused by:

    p : most_general_class_wide_access_type := new some_child_object
    q : most_general_class_wide_access_type := new some_other_child_object
...
    dispatchable_op(p); -- dispatch to some_child_object version
    dispatchable_op(q); -- dispatch to some_other_child_object version

??

If not, why would the above "super" example be confusing?

Also, I think someone else pointed out a while ago that using subtypes
(and maybe package renames) will make you immune to code "ripple effects"
if you change the parent:

    with p
    ...;
    package parents renames p;
    subtype parent is parents.t;
    type x is new parent;
...

Maybe news lost this...


/Jon
    
-- 
Jon Anthony
Organon Motives, Inc.
Belmont, MA 02178
617.484.3383
jsa@organon.com