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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Ada 2012 : aliased parameters ?
Date: Mon, 25 Apr 2011 02:19:35 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Date: 2011-04-25T02:19:35-05:00
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"Florian Weimer" <fw@deneb.enyo.de> wrote in message 
news:87aafgerez.fsf@mid.deneb.enyo.de...
>* Randy Brukardt:
>
>> The motivating case is to make the containers better. Ada 2012 adds the
>> following to all of the containers:
>>
>>     function Reference (Container : aliased in out Vector; Position : in 
>> Cursor)
>>        return Reference_Type;
>>
>> where Reference_Type is defined as:
>>
>>     type Reference_Type (Element : not null access Element_Type) is 
>> private
>>        with
>>           Implicit_Dereference => Element;
>
> Is it necessary that Element is a discriminant?

Yes, because access discriminants have special accessibility rules which 
happen to have the right effect.

>  If the aliased business works with fields, you could write something like 
> this:
>
>  type String_Reference is record
>     Data : access String;
>  end record;

You can write this, but you lose all accessibility checking if you do. The 
accessibility of a normal component is that of the type, which is typically 
library level. OTOH, a discriminant in a returned object has the 
accessibility of the point of call; combined with the rules for aliased 
parameters, such a discriminant will always succeed (no runtime checks or 
overhead needed) and will always be safe (can't copy it into anything with a 
longer lifetime, which is essentially anything).

I'm no fan of accessibility checks, but in this case at least they don't get 
in the way beyond preventing operations that we don't want to allow in the 
first place.

>In any case, it seems to me that the definition of "master" in 7.6.1 needs 
>updating.

It did, but only for bugs. The access discriminant semantics is from Ada 95, 
although it was never defined properly (probably still isn't, although not 
for the lack to trying). We've just found a good use for the strange 
semantics.

                              Randy.