Re: Question on Ada.Strings.Unbounded.Slice

[kst@thomsoft.com (Keith Thompson)]

In <DuE7D3.194@thomsoft.com> bn@alsys.com (Brian Nettleton @pulsar) writes:
> While building David Wheeler's "small" program (part of the Lovelace
> tutorial) I came across a problem with Ada.Strings.Unbounded.Slice.
> 
> I'm getting Index_error when the High > Length(Source) + 1 (where
> High, Length and Source are as defined in spec of Ada.Strings.Unbounded).

[example deleted]

> RM A.4.4:101 states Slice "propagates Index_Error if Low > 
> Length(Source) + 1."  But the RM is silent on what happens when
> High > Length(Source) + 1!  Is this an oversight in the RM, or
> by silence does the RM mean to imply that Index_Error is only
> raised when Low exceeds the bounds of the source?  Is it implementation
> dependent what happens when High > Length(Source) + 1?

According to the current (unapproved) version of AI95-00128, "A call to
Bounded.Slice with High > Length(Source) is equivalent to a call with
High = Length(Source)."  In other words, a slice that overlaps the end
of the string is silently truncated.

Personally, I disagree with this; I think the Slice function should
correspond to the predefined slice operation for type String (raising
Index_Error when a slice would raise Constraint_Error).  At least two
existing implementations do it this way.  I've just submitted a comment
to this effect to the ada-comment mailing list; we'll see what happens.

Incidentally, the problem occurs when High > Length(Source), not just
when High > Length(Source) + 1.

AI95-00128 is one of many commentaries on the Ada 95 standard; they're
available at

    <ftp://sw-eng.falls-church.va.us/public/AdaIC/standards/95com/ada-issues>

Note that these commentaries have little or no official standing until
they've gone through an approval process.  Someone more directly involved
than I am may wish to comment further on this process.

-- 
Keith Thompson (The_Other_Keith) kst@thomsoft.com <*>
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