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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: What is X'Address of an empty array?
Date: Tue, 31 May 2022 16:35:49 -0500
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Aliased objects should never have zero-size in Ada (or, at least, ever be 
allocated at the same address). I believe that is because of the required 
result of the equality operator. Specifically:

    type NUL is array (1..0) of Integer;
    A, B : aliased NUL;
    type PNul is access Nul;
    PA : PNul := A'Access; PB : PNul := B'Access;

    if PA = PB then
         Report.Failed ("Access to two distinct objects can not be equal");
    end if;

If an object is not aliased, it is undefined whether 'Address will work 
reliably with it (it probably does in GNAT, it might not in Janus/Ada, etc.) 
If the objects ARE aliased, then 'Address works essentially the same as 
'Access.

I personally find this a bit of overspecification in Ada, but since 
zero-size objects are unusual, no one has thought it worth going through the 
effort to change. (And of course such a change would complicate static 
analysis.) We (the ARG) did discuss this topic at one point (I don't have 
the AI number at hand).

                                   Randy.




"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:t7514c$11tj$1@gioia.aioe.org...
>I have a language lawyering question. According to ARM X'Address is the 
>address of the first array element. What is the address of empty array?
>
> In the case of an array with bounds it could be the address following the 
> bounds.
>
> But what about a definite empty array? Of zero length (and presumably zero 
> size). Would the compiler have to invent some address?
>
> P.S. With GNAT:
>
>    type NUL is array (1..0) of Integer;
>    S : NUL;
>
> S'Size is 8 and it has some address that holds the byte.
>
> Talking about the dark matter in our Universe. This is what empty arrays 
> are constructed of! (:-))
>
> -- 
> Regards,
> Dmitry A. Kazakov
> http://www.dmitry-kazakov.de