Re: unsigned type

[Adam Beneschan <adam@irvine.com>]

On Jun 30, 4:48 pm, a...@anon.org (anon) wrote:
> Read RM 3.5  ( 4 )
>
> Is "A ( 0 .. 0 )" an example of null range.  By the definition in RM 3.5
> ( 4 ), the Right side range (index) must be less than the Left side, so
> "A ( 0.. 0 )" is not a valid null range statement. So, this statement
> should generate a compiler or runtime error, because either range is not
> a subset of the range for Strings.

OK, I think I've finally figured out why we're having this confusing
argument.  It goes way back to this post of yours:

>> For Strings:
>>                          --  'A' is a zero length string, A'Last = 0, and
>>                          --  put_line  ( A ( A'First .. A'Last ) ) ;
>>                          --  does not raise an Constraint_Error even though in
>>                          --  this case it translate to:
>>                          --  put_line  ( A ( 0 .. 0 ) ) ;
>>  A : String := ""        ;

It does not translate to A (0..0); it translates to A (1..0).  If A is
declared as in your example above, A'First will be 1 and A'Last will
be 0.  Try it (try declaring A like that and outputting A'First and
A'Last).  It looks like everyone else missed this original error of
yours, which has apparently led to some confusion.

In this case, A'First..A'Last, which is 1..0, is compatible with the
subtype because it's a null range, and null ranges are compatible with
the subtype even when the range bounds don't actually belong to the
subtype.  0..0 is not compatible with the subtype, but you cannot
declare a string with that index range unless you try to do it
explicitly:

   A : String (0..0);

and then you *will* get a Constraint_Error.

So your later assertion that follows:

>> Since you can have zero length string , the index is Natual instead of Positive,
>> because zero is in Natural but is not define in Positive. Even though the
>> Standard package define String index as a Positive type. (GNAT)

is wrong.  The index range is Positive, but null ranges don't have to
meet that constraint.  They don't have to be Natural, either.  This is
legal and will not raise an exception at runtime:

   B : String (-9 .. -10) := "";

Hope this clears everything up.

                                  -- Adam