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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Assigning a "subclass" instance to a "superclass" variable
Date: Sat, 7 Apr 2012 09:06:14 +0200
Organization: cbb software GmbH
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Date: 2012-04-07T09:06:14+02:00
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On Fri, 6 Apr 2012 23:13:40 +0000 (UTC), deuteros wrote:

> I want something like a constructor in my child classes:
> 
>     	function Create_Expression (tokens, executedtokens : Vector) return Expression is
>     	    	E : Expression
>     	begin
>     	    	E.tokens := tokens;
>     	    	E.executedtokens := executedtokens;
>     	    	return E;
>     	end Create_Expression;
> 
> Now if the Expression package must have its own tokens and executedtokens
> I assume this would be the way to do it but if not then how would this
> work?

Then the parent type (Statement) should not have them in its
implementation. If children have token types of their own (a parallel types
hierarchy), you should provide an abstract interface to access
child-specific tokens. E.g.

   function Get_Token (S : Statement; Index : Token_Index_Type)
      return Token_Type'Class is abstract;

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