Re: Extension of non-limited type needs limited component

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

On Fri, 15 Nov 2002 00:30:03 GMT, Robert A Duff
<bobduff@shell01.TheWorld.com> wrote:

>Dmitry A. Kazakov <mailbox@dmitry-kazakov.de> writes:
>
>> 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.
>
>Compile time errors are good.  Exceptions (run-time errors) are evil.

Absolutely

>This is not SmallTalk, where you can dispatch to a method that does not
>exist.

Sure? And what about this:

type A is tagged ...
procedure Foo (L, R : A);

type B is new A with ....

X : A'Class := Some_A;
Y : A'Class := Some_B;

Foo (X, Y); -- Oops, dispatching to nowhere!

[ Not that I argue for SmallTalk, of course (:-)) ]

Then disallowing is already allowed for non-tagged types =
"is abstract". 

Further one can always override with a subroutine that throws
Program_Error. Is that better? IMO it is the same. The only real
difference is that one cannot get rid of overloading of the undesired
operation. What is actually required. I think that there could be
situations where it would be useful to disallow an operation in the
public interface and to override it with something working in the
private one.

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