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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: A smaller self contained test case. Was: Compiler Bug or what I'm
 doing wrong?
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Date: Sun, 25 Jun 2006 09:59:35 +0200
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Date: 2006-06-25T09:59:33+02:00
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On 24 Jun 2006 22:22:30 +0200, M E Leypold wrote:

> I followed Barnes' advice here: "wether it is appropriate to use a
> variant or tagged type, the key consideration is mutability". 

True [mutability of class instances] Handles to tagged types is another way
to have mutable class-wide.
 
>>   procedure Get_Record is
>>      F : File_Type;
>>      S : Stream_Access;
>>   begin
>>      Open (F, Mode => In_File, Name => "tmp-bug3" );
>>      S := Stream (F);
>>      for I in 1 .. 400 loop
>>         declare
>>            R : Customer_Description renames
>>                   Customer_Description'Input (S);
> 
> Oops. My, that is a dirty trick -- but how do I return R? Hm, yes I
> can see how that would work. Perhaps :-).  I'll see how I can fit that
> into the database module.

Hey, it is not a trick, but a nice pattern! I am using it quite often, and
count it for a good style.

Anyway, the following will work as well:

  procedure Get_Record is
     F : File_Type;
     S : Stream_Access;
     R : Customer_Description;
  begin
     Open (F, Mode => In_File, Name => "tmp-bug3" );
     S := Stream (F);
     for I in 1 .. 400 loop
        R := Customer_Description'Input (S);
        Ada.Text_IO.Put_Line ("Read" & Integer'Image (I));
     end loop;
     Close (F);
  end;

As far as I can tell, the bug is not in finalization. It is rather in
absence of. It seems that GNAT does *not* make it in Read when the
discriminant gets changed *by* Read. Then, later, another finalization
discovers rubbish. That never can happen when Input is used. So, it is safe
to use Input in all combinations. Just a guess.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de