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From: "Matthew Heaney" <mheaney@on2.com>
Newsgroups: comp.lang.ada
Subject: Re: generic question
Date: 21 Nov 2006 07:47:26 -0800
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Matthew Heaney wrote:
> "Alex R. Mosteo" <devnull@mailinator.com> writes:
>
> What you did above would be the idiom when you using the package itself to
> control allocation of objects:

Actually, I might do this differently for tagged vs. non-tagged types:

package P is
  type T (<>) is limited private;
  type T_Access is access T;
  function New_T return T_Access;
  procedure Free (X : in out T_Access);
...
end P;

At least this way you can set the pointer value to null on return from
Free.  For tagged types I'd probably do something like:

package Q is
  type T (<>) is tagged limited private;
  function New_T return not null access T;
  procedure Free (X : access T'Class);
...
private
  procedure Do_Free (X : access T);
...
end;

and then implement Free as a dispatching call to Do_Free, which can be
overridden by types that derive from T.

The reason for declaring Free with type T'Class is that if it were
declared as type T, then Free would be primitive for the type.  That
would make X a controlling operand and hence would have to be non-null.
 By declaring Free with type T'Class, the operation is no longer
primitive, X is not controlling, and so you can pass a null access
value without error.  You do lose the ability to set the access value
to null on return, however.  (Yes, you could use named access types,
but that defeats the implicit conversions that happen when you use
anonymous access types.)

In general when you're dealing with hierarchies of tagged types then
use anonymous access types, since this facilitates converting among
types in the hierarchy.