From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00 autolearn=ham
	autolearn_force=no version=3.4.4
X-Google-Thread: 103376,9e87e366cae15081
X-Google-Attributes: gid103376,domainid0,public,usenet
X-Google-Language: ENGLISH,ASCII-7-bit
Path: 
 g2news1.google.com!postnews.google.com!c19g2000prf.googlegroups.com!not-for-mail
From: "jimmaureenrogers@worldnet.att.net" <jimmaureenrogers@worldnet.att.net>
Newsgroups: comp.lang.ada
Subject: Re: if Restrict_Func /= null confusion
Date: Sun, 30 Mar 2008 14:34:31 -0700 (PDT)
Organization: http://groups.google.com
Message-ID: 
 <f478caea-daec-4722-8608-82204f89a445@c19g2000prf.googlegroups.com>
References: 
 <95f36a62-b94c-4211-896a-300fa98932b7@x41g2000hsb.googlegroups.com>
NNTP-Posting-Host: 75.70.240.233
Mime-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
X-Trace: posting.google.com 1206912871 25686 127.0.0.1 (30 Mar 2008 21:34:31
 GMT)
X-Complaints-To: groups-abuse@google.com
NNTP-Posting-Date: Sun, 30 Mar 2008 21:34:31 +0000 (UTC)
Complaints-To: groups-abuse@google.com
Injection-Info: c19g2000prf.googlegroups.com; posting-host=75.70.240.233;
	posting-account=fZH-XgkAAADP-Rf8L8ppyFIdKUfh90k4
User-Agent: G2/1.0
X-HTTP-UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US;
 rv:1.8.1.13)
	Gecko/20080311 Firefox/2.0.0.13,gzip(gfe),gzip(gfe)
Xref: g2news1.google.com comp.lang.ada:20653
Date: 2008-03-30T14:34:31-07:00
List-Id: <comp.lang.ada>

On Mar 30, 2:58 pm, ma740...@gmail.com wrote:
> Well folks its been at least 4 years since I've perused and/or written
> Ada source.  That aside I'm perusing source code written in Ada.   So
> consider:
>
> -- used to restrict a variable within a range
> -- for example: -PI to +PI
> type Restrict_Function is access function (X: Real4) return Real4;
>
> type Filt_Data is
>   record
>      Restrict_Func   : Restrict_Function ;
>
>   end record ;
>
>    --  within a procedure we have
>   Fdata             : Filt_Data;
>   Restrict_Func : Restrict_Function := FData.Restrict_Func ;
>
>   if  Restrict_Func /= null then
>    -- stuff
>   endif
>
> At issue:  I'm not following the conditional logic 'if (Restrict_Func /
> = null)'.  Not understanding the impetus behind the check for null.

Type Restrict_Function is an access type. It is possible for an
instance of Restrict_Function to have a null value. This kind of
problem can occur if you are creating an array of Restrict_Function
and only some of the elements of the array are set to non-null values.
You will encounter a run-time error if you try to dereference a null
access value.

Jim Rogers