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Date: Thu, 28 May 2009 15:00:12 +0200
From: Georg Bauhaus <rm.dash-bauhaus@futureapps.de>
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Subject: Re: Allocation questions
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Date: 2009-05-28T15:00:13+02:00
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Ludovic Brenta schrieb:
> xavier grave:
>>> Or you could use a discriminant:
>>>    type Samples (No_Of_Samples : Natural := 0) is
>>>       record
>>>          S : array (1 .. No_Of_Samples) of Sample;
>>>       end record;
>> My two cents about variant discriminant : IMHO one should avoid to
>> declare No_Of_Samples as Natural and with a default value, because as
>> far as I remember some compilers will book for S (1 .. Natural'Last) of
>> Sample which can be a lot...
>>
>> May be some compiler guru can confirm or infirm this ?
> 
> I confirm this with GNAT even though I'm no guru. Well spotted,
> Xavier.

Using GNAT and -fstack-check, this will stem towards the
stack limit (Dummy just declares Samples records of
increasing No_Of_Samples):

with Ada.Text_IO;

procedure GNAT_Varrec is

   type Sample is new Float range 0.0 .. 1.0;
   type Run is array (Natural range <>) of Sample;

   type Samples (No_Of_Samples : Natural := 0) is
      record
         S : Run (1 .. No_Of_Samples);
      end record;

   procedure Dummy (N: Natural) is
      Scratch: Samples(No_Of_Samples => N);
   begin
      if N in Scratch.S'Range then
         Scratch.S(N) := 0.5;
      end if;
   end Dummy;

   K : Natural;

   package Text_IO renames Ada.Text_IO;
begin
   K := 0;
   while K < Natural'Last loop
      Text_IO.Put_Line("Size is " &
	Natural'Image(K * Sample'Size / 8) & " octets");
      Text_IO.Flush;
      Dummy(K);
      K := K + 4096;
   end loop;
end GNAT_Varrec;