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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Efficient Sequential Access to Arrays
Date: Thu, 26 Jul 2012 15:46:04 +0200
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Date: 2012-07-26T15:46:04+02:00
List-Id: <comp.lang.ada>

On Thu, 26 Jul 2012 05:32:32 -0700 (PDT), Keean Schupke wrote:

> So if I do
> 
> if X'Valid then
>     Y(X) -- if X is the same type as the index of Y then no need to range check.
> end if
> 
> If I explicitly check 'Valid, the compiler should be able to omit some
> implicit range checks, as 'Valid is an explicit range check.

No. You do it this way:

declare
   subtype Y_Index is Integer range Y'Range;
   Safe_X : Y_Index := X;
begin
   ... Y (Safe_X) ... -- The compiler has here information to
                           -- eliminate check, though not required to
end;

P.S. The attribute 'Valid is used when there is a danger that the compiler
hasn't checked the argument for whatever reason. Usually it is after
Unchecked_Conversion.

Some would disagree, but IMO in a well-written Ada program you should never
ever need 'Valid. Don't do Unchecked_Conversion with doubtful outcome.
Don't use stream attributes for constrained types. Don't use for X'Address
use Z'Address for the purpose of conversion etc.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de