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From: mheaney@ni.net (Matthew Heaney)
Subject: Re: Ada to C/C++ translator needed
Date: 1996/09/30
Message-ID: <mheaney-ya023180003009960805430001@news.ni.net>#1/1
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references: <32499FA0.4B5E@magic.fr> <52e5t5$m28@btmpjg.god.bel.alcatel.be>
 <52feul$os2@goanna.cs.rmit.edu.au> <01bbad6e$67743f20$32ee6fcf@timhome2>
 <52ltk5$qlf@news1.halcyon.com> <01bbae25$67c669a0$32ee6fcf@timhome2>
 <dewar.844055713@schonberg> <01bbae8f$dffbd440$32ee6fcf@timhome2>
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organization: Estormza Software
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newsgroups: comp.lang.c,comp.lang.ada
Date: 1996-09-30T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <01bbae8f$dffbd440$32ee6fcf@timhome2>, "Tim Behrendsen"
<tim@airshields.com> wrote:

>I know that Ada does run-time checks of array bounds that C
>doesn't do; that could be one source of loss of performance.
>Does Ada do any other run-time checks that C doesn't do?

Not necessarily.  Using a subtype actually *turns off* type checking during
iteration through an array.  

If I did this

   type AT is array (Positive range <>) of T;

   O : AT (1 .. 10);
begin
   for I in 1 .. 10 loop
      O (I) :- ...

Then I might very well incur a check.  But by asserting a more specific
type for the loop index, no checks will be made inside the loop:

   for I in O'Range loop
      O (I)   -- no check, since I is guaranteed to to be in the range of O

Or

   subtype Index is Positive range 1 .. 10;

   O : AT (Index);
begin
   for I in Index loop
      O (I)  -- no check required inside the loop

If I didn't want to iterate through all the items in O, then I (the human)
would still include a subtype for I (the index):

   O : AT (Index);
begin
   for I in Index range 2 .. 8 loop
      O (I)...

Here you are asserting that I lies in the index subtype of O.  The checking
of that assertion need only take place once, on entry to the loop.

It's a common misunderstanding that a subtype puts checks in, but very
often just the opposite is true.  The idea is the give the compiler as much
information as you can when declaring the index that dereferences an array
object.  For example:

   O : AT (1 .. 10);
   I : Positive range O'Range; 
begin
   ...
   O (I) ...   -- no check required

or

   subtype Index is Positive range 1 .. 10;
   O : AT (Index);
   I : Index;
begin
   ...
   O (I) ...

>-- Tim Behrendsen (tim@a-sis.com)

Matt

--------------------------------------------------------------------
Matthew Heaney
Software Development Consultant
mheaney@ni.net
(818) 985-1271