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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 02:21:47 +0300
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Xref: news.eternal-september.org comp.lang.ada:47626
Date: 2017-08-07T02:21:47+03:00
List-Id: <comp.lang.ada>

Victor Porton wrote:

> 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;

I think it should be access parameter, because (as of Ada2005 and above) 
this allows passing null, what may be useful in some situations.

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