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From: Lutz Donnerhacke <lutz@iks-jena.de>
Newsgroups: comp.lang.ada
Subject: Re: Limited_Controlled types as 'out' arguments
Date: Wed, 30 Jul 2003 12:32:17 +0000 (UTC)
Organization: IKS GmbH Jena
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* Dmitry A Kazakov wrote:
> On Wed, 30 Jul 2003 11:31:03 +0000 (UTC), Lutz Donnerhacke
>>   procedure Copy(to : out Test; from : Test);
>>
>>Clearly, the variable 0 and 3 are never finalized. How implement I this
>>correctly (without refering to the rosen trick).
>
> 1. Why do you think that it is incorrect? Copy does not destruct the
>    target, so why should it call Finalize?

In my implementation, the type Test contains an array_access, I have to
deallocate, before assigning a new value. I can not deallocate this access
variable, because the procedure Free (unchecked_deallocate) requires an
'in out' Parameter. But I only have an 'out' paramter. So I can't read it,
before the first write.

In short: The former contents (despite discriminates) of an 'out' parameter
are not available inside the procedure. So the former contents are not
finalizable inside this procedure.

> procedure Copy (To : in out Test'Class; From : Test'Class) is
> begin
>    Destruct (To);
>    Construct (To, From);
> end Copy;

Fine, but try this with the prototype:
  procedure Copy (To : out Test'Class; From : Test'Class);

> Beware, true multiple dispatch is not supported in Ada. So if you want
> to copy objects of different types you have to simulate MD.

Wrong answer. Not every problem is MI related.