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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Unchecked_Deallocation with tagged class type.
Date: Tue, 14 Nov 2023 23:42:59 +0100
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On 2023-11-14 22:11, Blady wrote:

> The following code present a Finalize procedure with a parameter of 
> access tagged class type in order to deallocate the memory of the given 
> parameter from the root tagged type TTA and his children.
> The same for TTB which is inherited from TTA. But this Finalize call 
> Finalize of TTA.
> It may be not the best idea.
> But let's see:
> 
> with Ada.Unchecked_Deallocation;
> procedure test_20231113_free_tag is
> 
>     type TTA is tagged record
>        AA : Integer;
>     end record;
>     type ATTA is access all TTA;
>     type CATTA is access all TTA'Class;
>     procedure Finalize (O : in out CATTA) is
>        procedure Free is new Ada.Unchecked_Deallocation (TTA, ATTA);
>     begin
>        Free (ATTA (O));
>     end Finalize;
> 
>     type TTB is new TTA with record
>        BB : Integer;
>     end record;
>     type ATTB is access all TTB;
>     type CATTB is access all TTB'Class;
>     procedure Finalize (O : in out CATTB) is
>     begin
>        Finalize (CATTA (O));
>     end Finalize;
> 
>     OA : CATTA := new TTA;
>     OB : CATTB := new TTB;
> 
> begin
>     Finalize (OA);
>     Finalize (OB);
> end test_20231113_free_tag;
> 
> The procedure Free is the instanciation of Unchecked_Deallocation with 
> the tagged type TTA.
> Thus the call "Finalize (OA);" deallocate the memory of object OA of 
> type access class TTA.
> 
> But what does "Finalize (OB);"?

Crashes your program. It is a bug. You should instantiate 
Unchecked_Deallocation with class-wide type if you pass a class-wide 
pointer.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de