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From: Victor Porton <porton@narod.ru>
Newsgroups: comp.lang.ada
Subject: Re: Limited object by reference or access argument?
Date: Mon, 07 Aug 2017 01:01:43 +0300
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Date: 2017-08-07T01:01:43+03:00
List-Id: <comp.lang.ada>

Victor Porton wrote:

> Suppose we have
> 
> type World_Type is tagged limited private;
> 
> type BNode_ID_Handler is abstract tagged limited private;
> 
> We need to set a bnode handler for a world object. Should the procedure
> signature be
> 
> procedure Set_Handler (World: World_Type;
>                        Handler: access BNode_ID_Handler'Class);
> 
> or
> 
> procedure Set_Handler (World: World_Type;
>                        Handler: BNode_ID_Handler'Class);
> 
> The later can work with limited BNode_ID_Handler type, because tagged and
> limited types are always passed by reference.
> 
> I prefer the second one, because with the first procedure signature it is
> possible to pass an access to an already finalized object what is an
> error, but want to ask you opinion which of the two procedure signatures
> is better.

It may work like this (in fact it is more complex):

type World_Type is tagged limited
   record
      Handler: access BNode_ID_Handler'Class;
   end;

procedure Set_Handler (World: World_Type;
                       Handler: BNode_ID_Handler'Class) is
begin
   World.Handler := Handler'Unchecked_Access;
end;


-- 
Victor Porton - http://portonvictor.org