Re: Limited_Controlled types as 'out' arguments

[Lutz Donnerhacke <lutz@iks-jena.de>]

* Dmitry A Kazakov wrote:
>>> 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. (:-))

It's not that funny, if you need those data to manage ressources. Managing
data which is possibly garbage is very hard.

>>Discriminants of 'out' types have mode 'in'. Always.
>
> Not always:
>
> type A (I : Integer := 10) is null record;

Ok.

>>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.

I still do not get it completely.

------------------------------------------------------------------------
with Ada.Finalization;

package t1 is
   type Char_Access is access Character;
   type Test is new Ada.Finalization.Limited_Controlled with record
      a : Char_Access;
   end record;
   procedure Initialize(o : in out Test);
   procedure Finalize(o : in out Test);
   procedure Set(o1, o2 : out Test; i : Test);
end t1;
------------------------------------------------------------------------
with Ada.Text_IO;
with System.Storage_Elements, System.Address_To_Access_Conversions;
with Unchecked_Deallocation;
use Ada.Text_IO;

package body t1 is
   procedure Debug (msg : String; p : Char_Access) is
      use System.Storage_Elements;
      package Convert is new System.Address_To_Access_Conversions(Character);
   begin
      Put_Line(msg &
        Integer_Address'Image(To_Integer(
            Convert.To_Address(Convert.Object_Pointer(p)))) &
        '(' & p.all & ')');
   end Debug;
   
   global : Character := '0';
   
   procedure Initialize(o : in out Test) is
   begin
      o.a := new Character'(global);
      Debug("Initializing", o.a);
      global := Character'Succ(global);
   end Initialize;
   
   procedure Free is new Unchecked_Deallocation(Character, Char_Access);
   procedure Finalize(o : in out Test) is
   begin
      Debug("Finalizing  ", o.a);
      Free(o.a);
   end Finalize;
   
   procedure Set(o1, o2 : out Test; i : Test) is
   begin
      Debug("Setting from", i.a);
      Debug("Setting o1  ", o1.a);
      Debug("Setting o2  ", o2.a);
      Finalize(o1);
      Finalize(o2);
      
      o1.a := new Character'(global);
      Debug("Setting to1 ", o1.a);
      global := Character'Succ(global);

      o2.a := new Character'(i.a.all);
      Debug("Setting to2 ", o2.a);
   end Set;
end t1;
------------------------------------------------------------------------
with t1;
use t1;

procedure t is
   a, b : Test;
begin
   Set(a, a, b);
end t;
------------------------------------------------------------------------

Results in:
Initializing 134630800(0)
Initializing 134630816(1)
Setting from 134630816(1)
Setting o1   134630800(0)
Setting o2   134630800(0)
Finalizing   134630800(0)
raised PROGRAM_ERROR : t1.adb:14
Finalizing   134630816(1)

I'am mad?