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From: Samuel Tardieu <sam@ada.eu.org>
Subject: Re: run-time type identification
Date: 1998/09/03
Message-ID: <m367f58j5g.fsf@zaphod.enst.fr>#1/1
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References: <01bdd72e$49ee66b0$f330ea9e@ukp03332>
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To: "Bob Fletcher" <bob@radge.globalnet.co.uk>
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Date: 1998-09-03T00:00:00+00:00
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>>>>> "Bob" == Bob Fletcher <bob@radge.globalnet.co.uk> writes:

Bob> It seems to me that this is something that would make a lot of
Bob> sense when dealing with class-wide access types, which can easily
Bob> be used to go up the object heirarchy, but not, as far as I know,
Bob> to go back down, (without messing about with
Bob> unchecked_conversion).

You can do it in Ada: (pseudo-code that compiles but is obviously incorrect)

package T is
   type A is tagged null record;
   type A2 is new A with null record;
   type A_Ptr is access all A'Class;
   Z : A_Ptr;
   Y : constant A2 := A2 (Z.all);
end T;

Of course, Constraint_Error will be raised if Z.all does not belong to 
A2'Class.

  Sam
-- 
Samuel Tardieu -- sam@ada.eu.org