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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Efficient Sequential Access to Arrays
Date: Mon, 30 Jul 2012 22:12:27 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Date: 2012-07-30T22:12:27-05:00
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"Keean Schupke" <keean.schupke@googlemail.com> wrote in message 
news:b18e5dd8-cd07-49df-8011-22e911dc1da0@googlegroups.com...
On Thursday, 26 July 2012 01:15:25 UTC+1, Randy Brukardt  wrote:
...
>> Ada scalar objects can be in a state known as &quot;invalid&quot; (see 
>> 13.9.1 in your
>> Ada Reference Manual). For integer types, this corresponds to a value out 
>> of
>> range. Ada compilers are always allowed to return invalid results from
>> scalar operations, with a few exceptions. So in general, an Ada compiler
>> does not actually need to make range checks on scalar objects. The only 
>> time
>> that a check on a scalar object *has* to be performed is when using the
>> value could lead to erroneous execution (such as in indexing out of 
>> range).
>
>This is an equivalent concept to nullable, so two questions:
>
>What is the literal for 'invalid', IE:
>
>X : Index := #INVALID -- we should be able to initialise to invalid.

A subtype doesn't necessarily have any invalid values, so this is impossible 
in general. For instance, you can't write this for type Integer.

Pragma Normalize_Scalars does what you want if that's possible, but it 
applies to all objects in a program. (There's no way to do use it on a 
case-by-case basis.)

>if X = #INVALID -- I want to be able to test for invalid.

if not X'Valid then

will do the trick.

                                         Randy.