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From: sjw <simon.j.wright@mac.com>
Newsgroups: comp.lang.ada
Subject: Re: Unconstrained Arrays
Date: Wed, 25 Mar 2009 14:11:57 -0700 (PDT)
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Date: 2009-03-25T14:11:57-07:00
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On Mar 20, 11:40=A0am, Jean-Pierre Rosen <ro...@adalog.fr> wrote:
> sjw a =E9crit :> There is a deep language-lawyerly reason (which I don't =
understand)
> > why an array like your My_Array can't be aliased (at any rate in
> > Ada95); you have to use the initialize-with-aggregate approach.
> > Perhaps that's what leads to the initialize-with-aggregate style.
>
> Here is an example:
>
> procedure essai is
> =A0 =A0type Acc is access all String;
> =A0 =A0V : Acc;
> =A0 =A0S1 : aliased String :=3D "abcd";
> =A0 =A0S2 : aliased String (1..4);
> begin
> =A0 =A0V :=3D S1'Access;
> =A0 =A0V :=3D S2'Access;
> end;
>
> And this is what GNAT says:
>
> essai.adb:5:04: warning: aliased object has explicit bounds
> essai.adb:5:04: warning: declare without bounds (and with explicit
> initialization)
> essai.adb:5:04: warning: for use with unconstrained access
> essai.adb:8:09: object subtype must statically match designated subtype
>
> The message is pretty explicit. The underlying reason has to do with
> dope vectors (i.e. how the bounds of an array are attached to the
> array). It might be different, depending on whether the array is
> /declared/ constrained or not (S1 is declared unconstrained, although
> constrained by initialization).
>
> For example, in the constrained case, the array might include a pointer
> to a structure that describes the subtype, while in the unconstrained
> case, the array might keep the bounds with the data. Therefore, a
> pointer to a constrained array is incompatible with a pointer to an
> unconstrained array.

That makes so much sense! I think my brain freezes when I see the
compiler quoting rules about statically matching, so I just remember
the first 2 lines of the message (as you say, pretty explicit) and
pass over the reason why in the last 2 lines. After all, GNAT is much
more likely to be right than I am.