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From: "Randy Brukardt" <randy@rrsoftware.com>
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Subject: Re: Ada.Containers.Vectors - querying multiple elements
Date: Thu, 28 Apr 2005 15:55:25 -0500
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Date: 2005-04-28T15:55:25-05:00
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"Jeffrey Carter" <spam@spam.com> wrote in message
news:0uYbe.542$BE3.229@newsread2.news.pas.earthlink.net...
...
> The definition of Last was driven by the decision to allow null
> unbounded arrays regardless of the base range of Index. Currently,
> unconstrained Ada array types can exist that don't have null arrays.
> IIRC, this will be corrected in Ada 0X; if you allow them for arrays,
> you ought to allow them for the unbounded-array abstraction.

It's not really corrected in Ada 2006; it has additional aggregates that
help, but the basic problem of having to mess with the bounds still exists:

     Obj : Some_Array (Enum'First ..
Enum'Val(Enum'Pos(Enum'First)+Some_Length-1);

doesn't work if Some_Length = 0 and there is no general way to fix that. You
just have to special case Some_Length = 0 and that is unpleasant.

The solution that Ada.Containers.Vector took to this problem is essentially
to ban instantiating it with subtypes where 'First = 'Base'First. (OK, you
can do the instantiation, but it will raise Constraint_Error.) One can argue
whether that is the best solution, but it seems clear that all of the
possible solutions are ugly.

                  Randy.