Re: Limited_Controlled types as 'out' arguments

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

On Wed, 30 Jul 2003 12:32:17 +0000 (UTC), Lutz Donnerhacke
<lutz@iks-jena.de> wrote:

>* 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.

You can in Ada 95.

>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);

This compiles with GNAT:
-----------------------
with Ada.Finalization;

package A is
   type Int_Ptr is access Integer;
   type Test is new Ada.Finalization.Limited_Controlled with record
      Ptr : Int_Ptr;
   end record;
   procedure Destruct (To : in out Test);
   procedure Construct (To : in out Test; From : Test);
   procedure Copy (To : out Test'Class; From : Test'Class);
end A;
---------------------
with Ada.Finalization;
with Ada.Unchecked_Deallocation;

package body A is
   procedure Free is
      new Ada.Unchecked_Deallocation (Integer, Int_Ptr);
   procedure Destruct (To : in out Test) is
   begin
      Free (To.Ptr);
   end Destruct;
   
   procedure Construct (To : in out Test; From : Test) is
   begin
      To.Ptr := From.Ptr;
   end Construct;
   
   procedure Copy (To : out Test'Class; From : Test'Class) is
   begin
      Destruct (To);
      Construct (To, From);
   end Copy;
end A;

>> 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.

MD /= MI. Multiple dispatch is when there are many controlling
arguments of different tags. Assignment / copy is a case. You have
source and target, and sometimes you want assign:

X : Local_String;
Y : Remote_String;

Copy (Y, X); -- Constraint_Error

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