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Date: Fri, 03 Jul 2009 23:24:20 +0200
From: Georg Bauhaus <rm.tsoh.plus-bug.bauhaus@maps.futureapps.de>
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Date: 2009-07-03T23:24:21+02:00
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anon wrote:

> Since the RM does not directly 
> give one permission to use an illegal index value, then one should assume 
> that all index must be valid before checking for a null array. 

To "assume" formal properties like index validity seems inadequate:
this is about index ranges, not index values, and about what the RM
has to say about the formal notion of a "null range" (as has now been
said many times).

> Actually, 
> Ada RM 0 ( 29 ) states that it is illegal and the program shall create 
> an exception if the program "access an array component by using an 
> incorrect index value".

There isn't any access to an array component through a null range.
For indexing an array component, one must have an index value.
There is no value in a null range.  Therefore, there is no
access to an array component.


> Also, a number of ACATS test suggest this as well 
> and the program below will illustrate this. With that stated from the RM 
> the program shall 
> 
>  if S'First in S'Range and then S'Last in S'Range then 
>    if S'First <= S'Last then
>      -- access/create an array bound by ( S'First .. S'Last )
>    else
>     -- return a null created array access
>    end if ;
>  else
>   raise Constraint_Error ;
>  end if ;
> 
> You have to look at the whole RM to find out if there are other sections 
> that may clarify a statements or allow an exception. But there is no 
> RM rules that allows an out of bounds index to be used.

Maybe the verse and illustrations given by J.-P. Rosen and
Adam Beneschan have slipped:  There are RM rules regarding
null ranges.  The construct "Greater .. Smaller" as used in, say,
initializing a null string refers to null ranges, so not to any
index value at all.