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From: Maciej Sobczak <see.my.homepage@gmail.com>
Newsgroups: comp.lang.ada
Subject: Limited use for limited with?
Date: Tue, 28 Sep 2010 00:37:04 -0700 (PDT)
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Date: 2010-09-28T00:37:04-07:00
List-Id: <comp.lang.ada>

I have found a problem with the intended application of limited with.

Consider a package specifying some object-oriented construct:

package Objects is

   type Object is interface;
   type Object_Access is access all Object'Class;

   procedure Do_Something (X : in out Object) is abstract;

end Objects;

In such cases I routinely define the XYZ_Access type and later use it
wherever pointers to class-wide XYZ are needed.

Now consider a package that uses the above in a limited way (pun
intended), where only pointers to class-wide type are needed:

with Objects;
package Object_Users is

    procedure Use_Object (X : Objects.Object_Access);

end Object_Users;

The problem is that is some cases it would be more convenient (or just
more self-documenting from the design perspective) to do limited with
instead, but unfortunately this makes Object_Access unavailable. It is
OK to use anonymous access type, at least in some cases like here:

    procedure Use_Object (X : access Objects.Object'Class);

but I find that uncomfortable - after all, the proper access type is
already defined for exactly this purpose.

In such cases, where the design intent is pretty clear (pass around
references to Objects) I find that limited with does not really bring
the functionality that it is supposed to provide. In some more
involving cases I was forced to introduce additional and completely
artificial packages, where limited with would be a perfect fit.

Any thoughts on this?

--
Maciej Sobczak * http://www.inspirel.com