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From: Martin Krischik <krischik@users.sourceforge.net>
Newsgroups: comp.lang.ada
Subject: Re: Dynamically tagged expression required
Date: Thu, 08 Dec 2005 20:17:18 +0100
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Date: 2005-12-08T20:17:18+01:00
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Maciej Sobczak wrote:

> Hi,
> 
> 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);

try:

        D : Shape'Class := C;

as well - it will work as well.
> 
> 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?

because it is an initialization.

> Why is (3) not allowed?

because it is an assignment.

Read:

http://en.wikibooks.org/wiki/Ada_Programming/Subtypes
http://en.wikibooks.org/wiki/Ada_Programming/Object_Orientation


Martin

-- 
mailto://krischik@users.sourceforge.net
Ada programming at: http://ada.krischik.com