From: "J-P. Rosen" <rosen@adalog.fr>
Subject: Re: Assignment access type with discriminants
Date: Thu, 23 Mar 2023 18:04:36 +0100 [thread overview]
Message-ID: <tvi0qt$17pev$1@dont-email.me> (raw)
In-Reply-To: <tveh6u$ihpu$1@dont-email.me>
Le 22/03/2023 à 10:19, Dmitry A. Kazakov a écrit :
> I stumbled on a curious fact.
>
> The value of an object with a discriminant can be changed to a value
> with a different discriminant if the type's discriminants are defaulted.
>
> Right?
>
> Wrong! Not through an access type!
>
(...)
> Is this a compiler bug or intentional language design? Any language
> lawyers?
>
An access value is always constrained by its initial value; this is
necessary because of constrained access subtypes. Here is a slightly
modified version of your example:
procedure Test is
type F is (F1, F2, F3);
type Foo (K : F := F1) is record
case K is
when F1 =>
X1 : Integer;
when F2 =>
X2 : Float;
when F3 =>
X3 : String (1..2);
end case;
end record;
type Foo_Ptr is access all Foo;
type Foo_Ptr2 is access Foo;
X : aliased Foo;
P : Foo_Ptr := X'Access;
PF2: Foo_PTR2 (F2);
begin
X := (F2, 1.0); -- OK
PF2 := new Foo (F2);
P := PF2.all'Access;
P.all := (F1, 3); -- Error!
end Test;
Without this rule, PF2.all would now designate a value whose
discriminant is F1!
--
J-P. Rosen
Adalog
2 rue du Docteur Lombard, 92441 Issy-les-Moulineaux CEDEX
https://www.adalog.fr https://www.adacontrol.fr
next prev parent reply other threads:[~2023-03-23 17:04 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-22 9:19 Assignment access type with discriminants Dmitry A. Kazakov
2023-03-22 9:31 ` Björn Lundin
2023-03-22 14:10 ` G.B.
2023-03-23 11:51 ` Dmitry A. Kazakov
2023-03-23 16:53 ` AdaMagica
2023-03-23 18:09 ` Niklas Holsti
2023-03-23 17:04 ` J-P. Rosen [this message]
2023-03-23 18:55 ` Niklas Holsti
2023-03-23 19:53 ` Dmitry A. Kazakov
2023-03-24 9:41 ` J-P. Rosen
2023-03-25 8:51 ` Randy Brukardt
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