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Newsgroups: comp.lang.ada
Date: Mon, 15 Oct 2018 09:05:47 -0700 (PDT)
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Subject: Re: In mode parameter in an assignment statement
From: Anh Vo <anhvofrcaus@gmail.com>
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Xref: reader02.eternal-september.org comp.lang.ada:54593
Date: 2018-10-15T09:05:47-07:00
List-Id: <comp.lang.ada>

On Saturday, October 13, 2018 at 1:38:03 AM UTC-7, Jeffrey R. Carter wrote:
> On 10/13/2018 02:48 AM, Anh Vo wrote:
> > 
> > I thought it should be rejected during compilation because in mode parameter is not allowed in an assignment statement. But, it was compiled successfully under GNAT obviously.
> > 
> > What did I miss? Thanks in advance.
> 
> Others have pointed out that the in-mode parameter, Region, is of an access 
> type. What may not be clear is why this makes a difference. It's important to 
> distinguish the access value (stored in Region) from its designated value 
> (Region.all). The access value is constant within the procedure; any attempt to 
> change it
> 
> Region := new Mapped_Region_Record;
> 
> would result in a compiler error. However, the access value is the only thing 
> that is an in-mode parameter and constant within the procedure. The designated 
> value, Region.all, is not constant and may be modified. That is what's happening 
> in the procedure. That Ada access types allow for implicit dereference and can 
> usually be used exactly the same way as the designated value can be confusing if 
> one is not aware that the type is an access type.
>

I got it. Thank you all for your replies specially for Jeffrey's detailed explanation.

Anh Vo