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From: mheaney@on2.com (Matthew Heaney)
Newsgroups: comp.lang.ada
Subject: Re: virtual destructors
Date: 23 Apr 2003 08:16:44 -0700
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Stephen Leake <Stephe.Leake@nasa.gov> wrote in message news:<u3ckacuwv.fsf@nasa.gov>...
> "kat-Zygfryd" <6667@wp.pl> writes:
> 
> Yes. The compiler ensures that Finalize is called. Users will forget
> to call Dispose.

Another possibility is to use a handle of some kind (a la the C++
auto_ptr class), that claims ownership of the access object, and
automatically calls Free when the handle object is itself finalized.

The Charles library has an Access_Control package that does just that,
e.g.

declare
   P1 : Pointer_Type; --from Charles.Access_Control
   P2 : Pointer_Type;
begin
   Initialize (P1, new T); --claim ownership
   Assign (Target => P2, Source => P1); --transfer ownership
   Op (+P2); --use access object
end; --P2 will Free object

You can even use this in a factory function:

function New_T return T_Access is
   P : Pointer_Type;
begin
   Initialize (P, new T); --claim ownership
   --do some stuff that may raise an exception
   return Release (P); --relinguish ownership
end;

If the factory function New_T raises an exception prior to reaching
the return, then pointer P will free the allocated object during its
finalization, because it still owns the object.

If we do reach the return statement, then we tell P to release the
object, so P does nothing during its finalization, and ownership of
the allocated object is transfered to the caller of New_T.

The party to whom New_T transfers ownership might indeed even be
another access control object:

declare
   P : Pointer_Type;
begin
   Initialize (P, Value => New_T);
   --now use +P
end; --free object during finalization of P

The Charles.Access_Control component is part of the Charles library,
available at my website:

http://home.earthlink.net/~matthewjheaney/charles/