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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 15:15:09 +0000 (UTC)
Organization: IKS GmbH Jena
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* Dmitry A Kazakov wrote:
> On Wed, 30 Jul 2003 14:25:46 +0000 (UTC), Lutz Donnerhacke
>>* Dmitry A Kazakov wrote:
>>> On Wed, 30 Jul 2003 12:32:17 +0000 (UTC), Lutz Donnerhacke
>>>>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.
>>>
>>> You can in Ada 95.
>>
>>Oops. Why? This is a clear data flow error, which should be avoided.
> 
> It is an interesting question. What could be alternatives
> 
> 1. to leave "out" parameters uninitialized. This is bad.

This is the most common implementation on most types. (most = unless you
define an Initialize procedure, default component data, or provide
initialization data)

> 2. to disallow reading from them, but then how to get discriminants?

Discriminants of 'out' types have mode 'in'. Always.

> 2.a. to disallow reading before first write except for discriminants,
> constraints and type tag.

Discriminants, ... can't be written on 'out' types.

> 2.b. return statement for "out"
> 
>    procedure Foo (X : out String; Y : Some'Class) is
>    begin
>       return
>       (  X => "Any text",
>          Y => Create_Some
>       );
>    end Foo;

No!

3) Finalize the current data of the 'limited out' parameter.
   Adjust the assigned data of the 'unlimited out' paramter after return.