From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00,FREEMAIL_FROM
	autolearn=unavailable autolearn_force=no version=3.4.4
Path: 
 eternal-september.org!reader01.eternal-september.org!reader02.eternal-september.org!news.eternal-september.org!news.eternal-september.org!feeder.eternal-september.org!aioe.org!.POSTED!not-for-mail
From: Victor Porton <porton@narod.ru>
Newsgroups: comp.lang.ada
Subject: Re: Limited object by reference or access argument?
Date: Mon, 07 Aug 2017 03:35:46 +0300
Organization: Aioe.org NNTP Server
Message-ID: <om8cl0$1q08$1@gioia.aioe.org>
References: <om80vg$1a68$1@gioia.aioe.org> <om83ke$1ejo$1@gioia.aioe.org>
 <om88a9$1kis$1@gioia.aioe.org>
NNTP-Posting-Host: qrWnZLr+FAc2xoElQM2hng.user.gioia.aioe.org
Mime-Version: 1.0
Content-Type: text/plain; charset="ISO-8859-1"
Content-Transfer-Encoding: 7Bit
X-Complaints-To: abuse@aioe.org
User-Agent: KNode/4.14.10
X-Notice: Filtered by postfilter v. 0.8.2
Xref: news.eternal-september.org comp.lang.ada:47627
Date: 2017-08-07T03:35:46+03:00
List-Id: <comp.lang.ada>

Victor Porton wrote:

> 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.

Another argument for access types is that if we switch to a non-limited 
type, copying the Handler object may not happen (and we should be sure that 
it is not copied by the compiler).

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