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From: rls@onondaga.steinmetz.ge.com (Rod Sprattling)
Newsgroups: comp.std.c++
Subject: Re: null references & dereferencing null pointers
Message-ID: <RLS.90Jul31125458@onondaga.steinmetz.ge.com>
Date: 31 Jul 90 17:54:58 GMT
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In-reply-to: jimad@microsoft.UUCP's message of 30 Jul 90 17:50:21 GMT


One use for dereferencing null pointers leaps to mind.  Consider this
definition, used in Intrinsics-based widgets in the X Window System:


Copyright 1987, 1988 by Digital Equipment Corporation, Maynard,
Massachusetts, and the Massachusetts Institute of Technology,
Cambridge, Massachusetts. All Rights Reserved.

#define XtOffset(type,field) \
        ((unsigned int) (((char *) (&(((type)NULL)->field))) - \
                         ((char *) NULL)))



It's used to calculate offsets into the elements of a struct, as
in this snippet from a fictional wristWatch widget:

static XtResource Resources[] = 
{
  { XtNwatchFaceColor,
      XtCFaceColor,
      XtRString,
      sizeof(char *), 
      XtOffset(GIWristWatchWidget, watch.face_color),
      XtRImmediate,
      "White"
  },
....


Can anyone else think of a situation where null dereferencing is
useful, besides compile-time calculation of offsets?


Rod
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Roderick Sprattling                      DialCom:  8*833-7054

sprattlingrl@crd.ge.com	                 rls@onondaga.crd.ge.com
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