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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: Wed, 15 Nov 2023 22:17:37 +0100
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On 2023-11-15 21:26, Blady wrote:
> Le 14/11/2023 à 23:42, Dmitry A. Kazakov a écrit :
>>> 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.
> 
> Thanks, I haven't considered this possibility.
> Note: the previous program built with GNAT FSF 13.2.0 ran without 
> exception.
> 
> I've changed:
> with Ada.Unchecked_Deallocation;
> procedure test_20231113_free_class 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'Class, CATTA);
>     begin
>        Free (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_class;
> 
> It runs without exception.
> One question remains about "Finalize (OB);":
> Which memory size is deallocated TTA'Size or TTB'Size?

It is a wrong question. The implementation of the pool may ignore size 
using the block size instead. Furthermore T'Size is not necessarily the 
size actually allocated.

Regarding Unchecked_Deallocation instantiated with a pointer to a 
class-wide object, consider it dispatching on the target object tag. 
Thus you can deallocate any object using any instance of 
Unchecked_Deallocation for any class-wide parent of, interfaces included.

So Finalize (OB) is OK.

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