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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Dynamically tagged expression required
Newsgroups: comp.lang.ada
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Date: Thu, 8 Dec 2005 15:52:06 +0100
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Date: 2005-12-08T15:52:06+01:00
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On Thu, 08 Dec 2005 14:50:37 +0100, Maciej Sobczak wrote:

> procedure Hello is
> 
>     type Shape is tagged null record;
>     type Triangle is new Shape with
>       record
>          SideLen : Positive;
>       end record;
> 
>     A : Shape;
>     B : Shape'Class := A;             -- (1)
>     C : Triangle := (SideLen => 7);
> 
> begin
> 
>     A := C;                           -- (2)
>     B := C;                           -- (3)
> 
> end Hello;
> 
> (2) does not compile, and this is what was expected.
> (3) does not compile, neither, beucase "dynamically tagged expression 
> required".
> 
> Interestingly, (1) is fine.
> 
> Why is (1) allowed?

It is not an assignment, but initialization.

> Why is (3) not allowed?

B := B; -- Legal
B := Shape'Class (A); -- Legal
B := Shape'Class (C); -- Legal, but raises Constraint_Error

You cannot change the tag of a class-wide object, so it would have little
sense anyway. However if ":=" were considered as a dispatching operation,
then (3) should be made legal.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de