Re: Extension of non-limited type needs limited component

[Dmitry A. Kazakov <mailbox@dmitry-kazakov.de>]

On Thu, 14 Nov 2002 13:33:07 +0100, "Jean-Pierre Rosen"
<rosen@adalog.fr> wrote:

>"Mike" <michael.jackson5@virgin.net> a �crit dans le message news:
>2dbd76f3.0211140126.5d233e41@posting.google.com...
>> How can assignment by upcasting the extended type possibly affect the
>> limited component?
>>
>Consider:
>
>   X : T'class := Some_Dynamic_Function (1);
>   Y : T'class := Some_Dynamic_Function (2);
>begin
>   X := Y;
>
>At compile time, we know nothing about the specific type of the values in X
>and Y, but assuming that T is not limited, the assignment is allowed. If X
>and Y happen to have different tags at run time, the assignment will raise
>Constraint_Error, but that's all.
>
>If you were allowed to extend T with limited components, you would
>effectively copy the limited components!

An LSP violation to put it short.

Which also raises an interesting question, whether T'Class should
always inherit "non-limitness" of T. Or more generally shouldn't one
have an ability to disallow primitive operations (like ":=" in case of
non-limited->limited mutation)?

Let we allow disallowing, then either

1. ":=" should dispatch in X:=Y to a disallowed operation, thus raise
an exception and so refuse to copy the limited components.

2. ":=" is class-wide in the right parameter, then it will dispatch
somewhere within its body and again an exception will happen if an
attempt to copy a limited component will be made..

---
Regards,
Dmitry Kazakov
www.dmitry-kazakov.de