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From: ka@socrates.hr.att.com (Kenneth Almquist)
Subject: Re: Java vs Ada 95 (Was Re: Once again,
 Ada absent from DoD SBIR solicitation)
Date: 1996/10/28
Message-ID: <5536en$2vk@nntpa.cb.lucent.com>#1/1
X-Deja-AN: 192915449
references: <ROGOFF.96Oct14095012@sccm.Stanford.EDU>
 <DzBM79.52n.0.-s@inmet.camb.inmet.com>
organization: Lucent Technologies, Columbus, Ohio
newsgroups: comp.lang.ada
Date: 1996-10-28T00:00:00+00:00
List-Id: <comp.lang.ada>


mg@dsd.camb.inmet.com (Mitch Gart) wrote:
> Calling superclass methods is easy in Java and hard in Ada:
>
>   type parent_obj is tagged record ...;
>   type parent_ptr is access all parent_obj;
>   procedure p(param: access parent_obj);
>
>   type child_obj is new parent_obj with ...;
>   type child_ptr is access all child_obj;
>   procedure p(param: access child_obj);
>
> now inside the child's p, to call the parent's p:
>
>   p(parent_obj(param.all)'access);
>
> is the way to do it.  Converting the pointer to parent_ptr won't work
> because the call will dispatch back to the child.

No it won't.  Dispatching only occurs when you use class-wide types.[1]

You don't explain why you are using access types.  Normally you would
write:

   type parent_obj is tagged record ...;
   procedure p(param: in out parent_obj);

   type child_obj is new parent_obj with ...;
   procedure p(param: in out child_obj);

Now inside the child's p, to call the parent's p:

   p(parent_obj(param));

This reads quite well (or at least it would if parent_obj had a meaningful
name).
					Kenneth Almquist


[1] You don't want to convert to parent_ptr because of the possibility
    of accessibility errors.  Instead use a pointer type declared in p:

procedure p(param: access child_obj) is
   type super is access all parent_obj;
begin
   ...
   p(super(param));  -- call the parent's p
   ...
end p;