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From: sabljak@cs.rmit.edu.au (Steve Sabljak)
Subject: Re: run-time type identification
Date: 1998/09/03
Message-ID: <6sm71d$nee$1@emu.cs.rmit.edu.au>#1/1
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Newsgroups: comp.lang.ada
Date: 1998-09-03T13:53:49+00:00
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sabljak@cs.rmit.edu.au (Steve Sabljak) writes:

>"Bob Fletcher" <bob@radge.globalnet.co.uk> writes:

>>Is there any equivalent in Ada 95 to the C++ "dynamic_cast" operator?

>>For example, say you have a class A, and a derived class A2.
>>In the package for class A there is also:
>>	type A_Ptr is access A'Class;
>>You can assign an access to an object of class A2 to a variable of type
>>A_Ptr, but, as far as I know, cannot then de-reference the A_Ptr in such a
>>way that the extra bits of class A2 are accessible.

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

>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;

Or even

begin
   Specialised := new A2'(X => 1, Y => 2);
   Generalised := Specialised;
   if Generalised.all in A2 then
	   Specialised_2 := A2_Ptr(Generalised);
   end if;
end Tag_Test;

cheers
-Steve
--
Steve Sabljak - RMIT - 3rd Year Computer Science  
e-mail: sabljak@cs.rmit.edu.au            
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