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From: mheaney@on2.com (Matthew Heaney)
Newsgroups: comp.lang.ada
Subject: Re: Disriminant question
Date: 7 Mar 2003 08:46:10 -0800
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 <slrnb6gvth.nt.lutz@taranis.iks-jena.de>
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Lutz Donnerhacke <lutz@iks-jena.de> wrote in message news:<slrnb6gvth.nt.lutz@taranis.iks-jena.de>...
> * prashna wrote:
> > How to change discriminant value dynamically?
> 
> This is impossible. You can't change the discriminant, because the
> instantiated variables does occupy different space. But you can set the
> discriminant dynamically.

The rule is that you can't change the discriminant if the object was
declared with an explicit discriminant value.

type RT (D : DT := DT'First) is 
   record
      case D is
         when DT'First =>
            null;

         when others =>
            null;
      end case;
   end record;

declare
  R1 : RT; --yes, can be changed

  R2 : RT := (D => DT'Last);  --yes, can be changed

  R3 : RT (DT'First); --no, cannot be changed

  R4 : RT (DT'First) := (D => DT'First); --no, can't be changed
begin
  R1 := (D => DT'Last);    --OK
  R2 := (D => DT'First);   --OK
end;

Basically, whenever you explicitly specify the discrimiant value in
the declaration (i.e. the "subtype mark"), then that constrains the
object, so that its discriminant cannot be changed.

This allows the compiler to allocate only as much stack space as is
needed for those components that apply to that discriminant value.

Note that in the case of R3 and R4 above, it doesn't matter that the
type has a default discriminant.  The fact that the object was
declared with an explicit value constrains the (sub)type.