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From: =?ISO-8859-1?Q?Hibou57_=28Yannick_Duch=EAne=29?=
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Newsgroups: comp.lang.ada
Subject: Re: unsigned type
Date: Sat, 11 Jul 2009 07:40:12 -0700 (PDT)
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Date: 2009-07-11T07:40:12-07:00
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On 29 juin, 10:00, a...@anon.org (anon) wrote:
>                           --  '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 := ""        ;
> 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.

The length of a string is Natural, the index type Positive, as long as
lowest bound is 1.

A zero length string does not have any valid index. Using strings,
unless I can assert the length is greater than zero, I always check
the length before any attemp to use its 'First and 'Last attributes.
Conceptually, when a string is zero length, at least one of its bound
is not in the valid range of its index type, and any way, none of its
bound would be a valid index in the string.

There is a precondition to access both bound or to access element of a
string by index : this precondition as that its length should be
greater than zero.

Printing a zero length string does not imply attempting to print
chracters in index range 1 .. 0, but may be instead properly handled
by simple not accessing the string and simply not printing anything,
providing the length has been previously known to be zero.