From mboxrd@z Thu Jan  1 00:00:00 1970
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From: Matthew Heaney <matthew_heaney@acm.org>
Subject: Re: run-time type identification
Date: 1998/09/03
Message-ID: <m3zpch4qn2.fsf@mheaney.ni.net>#1/1
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Sender: matt@mheaney.ni.net
References: <01bdd72e$49ee66b0$f330ea9e@ukp03332>
 <6sm537$agc$1@emu.cs.rmit.edu.au>
NNTP-Posting-Date: Thu, 03 Sep 1998 07:05:57 PDT
Newsgroups: comp.lang.ada
Date: 1998-09-03T00:00:00+00:00
List-Id: <comp.lang.ada>

sabljak@cs.rmit.edu.au (Steve Sabljak) writes:

> You can do run-time type identification like this
> 
> with Ada.Tags; use Ada.Tags;
> with A;        use A;
> 
> procedure Tag_Test is
> 
>    Generalised   : A_Ptr;   
>    Specialised   : A2_Ptr;   
>    Specialised_2 : A2_Ptr;   
> 
> begin
>    Specialised := new A2'(X => 1, Y => 2);
>    Generalised := Specialised;
>    if Generalised'Tag = A2'Tag then
> 	   Specialised_2 := A2_Ptr(Generalised);
>    end if;
> end Tag_Test;

I prefer to just use a membership test.

To test for membership in the specific type A2:

   if Generalized.all in A2 then

To test for membership in the class-wide type A2'Class:

   if Generalized.all in A2'Class then


Don't bother with the Tag attribute or the package Ada.Tags.

You don't really need the tag test to do the type cast.  If the actual
type of the source object (Generalized.all) isn't in A2'Class, then the
type cast "A2_Ptr(Generalized)" will raise Constraint_Error: