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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Equivalent of dynamic_cast (downcast) for tagged types
Newsgroups: comp.lang.ada
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Date: Thu, 27 Jan 2011 17:18:03 +0100
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Date: 2011-01-27T17:18:04+01:00
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On Thu, 27 Jan 2011 07:46:33 -0800 (PST), Maciej Sobczak wrote:

> Unfortunately I need to HACK some equivalent of dynamic_cast in Ada.
> 
> There is a limited tagged type T deriving from Limited_Controlled and
> Some_Interface as well.
> There is also a procedure like this:
> 
> procedure Foo (X : in Some_Interface'Class);
> 
> Inside Foo I would like to check if X is indeed T and if so, I would
> like to dynamic_cast to it and execute some operation on T. That
> operation is in fact declared in Some_Interface anyway, but I would
> like to avoid the dispatching call. I would also like to avoid any
> potential data copying, so access types are welcome.

You don't need access type, tagged types are by-reference anyway.

> In C++ it would look like this:
> 
> void foo(some_interface & x)
> {
>     T * p = dynamic_cast<T*>(&x);
>     if (p != NULL)
>     {
>         // indeed x is of type T
>         p->do_something_for_me();
>     }
> }

   if X in T'Class then
      Do_Something (T (X));  -- No dispatch
   end if;

The above is a bad idea, because X might be a descendant of T which has
overridden Do_Something. So you might be looking for this

   if X'Tag = T'Tag then -- X is of T
      Do_Something (T (X));  -- No dispatch
   end if;

> If you say that I should not do this because it is not good practice,
> then I will quickly answer that GNAT should not randomly call Adjust
> on my limited (!) objects.

Adjust is not called on limited objects.

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