Re: Ada.Containers.Vectors - querying multiple elements

[Jeffrey Carter <spam@spam.com>]

Randy Brukardt wrote:

> 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.

Right, but you can say

Obj : Some_Array := Some_Array'(<>);

(I hope I've remembered the syntax correctly) and Obj'Length = 0; there 
was no way to do this before if Enum'First = Enum'Last. And you should 
be able to convert Obj into a corresponding unbounded array.

> 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.

Yes. I think this solution is uglier than having Last = First if Length = 0.

-- 
Jeff Carter
"Beyond 100,000 lines of code you
should probably be coding in Ada."
P. J. Plauger
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