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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Need help for constrained type
Date: Thu, 06 Feb 2014 20:37:30 +0200
Organization: Tidorum Ltd
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Date: 2014-02-06T20:37:30+02:00
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On 14-02-06 20:03 , adambeneschan@gmail.com wrote:
> On Thursday, February 6, 2014 9:30:32 AM UTC-8, Niklas Holsti wrote:
> 
>> (If you want to allow an empty Usage_Info, indexed 0..-1 for
>> example, you must of course declare Memory_Block.last using some
>> range which includes negative numbers, for example Integer.)
> 
> That actually isn't true.

It is here, I think, because the upper bound comes from a discriminant,
where subtypes are checked more strictly. And the lower bound is fixed
to zero, so only a negative upper bound makes an empty range.

> The bounds of an empty range do *not* need
> to be within the subrange.  Here, Natural is a subrange 0 ..
> Integer'Last, but Integer is the base type, and as long as the bounds
> belong to the base type, it's OK.

Agreed. But if I have

  type Memory_Block (last : Natural) is
    record
      ...
      Usage_Info : Mem_Array (0..last);
    end record;

then I'm pretty sure that the declaration

   M : Memory_Block (last => -1);

will be rejected, because -1 is not in Natural. GNAT says:

mb.adb:15:30: warning: static value out of range of type "Standard.Natural"
mb.adb:15:30: warning: "Constraint_Error" will be raised at run time

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
      .      @       .