Re: Limited_Controlled types as 'out' arguments

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

On Wed, 30 Jul 2003 14:25:46 +0000 (UTC), Lutz Donnerhacke
<lutz@iks-jena.de> wrote:

>* 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.

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

2.a. to disallow reading before first write except for discriminants,
constraints and type tag. Well, how many pages would it take to
consistently describe something like that in ARM? (:-)) Add here is
some run-time checks burden.

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;

looks awful, and after all how to implement unconstrained "out"
parameters?

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