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From: "kat-Zygfryd" <6667@wp.pl>
Newsgroups: comp.lang.ada
Subject: Re: virtual destructors
Date: Tue, 22 Apr 2003 19:40:06 +0200
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"Matthew Heaney" <mheaney@on2.com> wrote in message
news:1ec946d1.0304220928.72c2acc8@posting.google.com...
> "kat-Zygfryd" <6667@wp.pl> wrote in message
news:<b83ivk$29l$1@news.onet.pl>...
> > How can I define a virtual destructor in Ada95?
>
> There are two ways.
>
> I. Use controlled types
>
> Have your type (privately) derived from Controlled, and then use
> Unchecked_Deallocation to delete it through a class-wide type:
>
> package P is
>
>    type T is tagged private;
>
>    type T_Class_Access is access all T'Class;
>
>    procedure Op (O : access T);
>
> private
>
>    type T is new Controlled with record
>       ...;
>    end record;
>
>    procedure Finalize (O : in out T);
>
> end P;
>
>
> package P.C is
>
>    type NT is new T with private;
>
> private
>
>    type NT is new T with record
>       ...;
>    end record;
>
>    procedure Finalize (O : in out NT);
>
> end P.C;
>
> procedure Free is
>   new Ada.Unchecked_Deallocation (T'Class, T_Class_Access);
>
> declare
>    O1 : T_Class_Access := new T;
>    O2 : T_Class_Access := new NT;
> begin
>    Free (O1);  --call P.Finalize
>    Free (O2);  --call P.C.Finalize
> end;
>
>
> What's happening here is that the Ada run-time system knows that types
> in T'Class are controlled, so that when you use
> Unchecked_Deallocation, it calls the Finalize operation for the object
> being deleted.
>
> So in this sense destructors are already dispatching in Ada95, and
> there's nothing special you need to do.
>
> Note that Finalize operations for a class are typically implemented by
> performing actions specific to the derived type, and then forwarding
> the call to the parent type, e.g.
>
> procedure Finalize (O : in out NT) is
> begin
>   --do NT stuff here
>   Finalize (T (O)); --"view conversion" to call T's Finalize
> end;

Thanks, I like that :) Hope it'll work for me

Zygfryd