Re: Limited_Controlled types as 'out' arguments

[Dmitry A. Kazakov <mailbox@dmitry-kazakov.de>]

On Wed, 30 Jul 2003 15:15:09 +0000 (UTC), Lutz Donnerhacke
<lutz@iks-jena.de> wrote:

>* 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)

No they all are initialized, but probably with a garbage. (:-))

A really uninialized object is *(cast *) malloc (). Fortunately, we
have no this evil in Ada.

>> 2. to disallow reading from them, but then how to get discriminants?
>
>Discriminants of 'out' types have mode 'in'. Always.

Not always:

type A (I : Integer := 10) is null record;   
procedure Foo (X : out A) is
begin
   X := (I => 5); -- Change the discriminant
end Foo;

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

It would be inconsistent. Adjust is called after a raw copy is made,
your subprogram uses a valid type interface instead. And you better
don't do it after Finalize [+ other destructors] were called. Consider
a task object as a member of your limited type.

What you actually describe is a copy constructor, not a subroutine.

---
Regards,
Dmitry Kazakov
www.dmitry-kazakov.de