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From: dsr@luke.mitre.org (Douglas S. Rand)
Newsgroups: comp.lang.c++
Subject: Re: testing this==0
Keywords: this
Message-ID: <111892@linus.mitre.org>
Date: 27 Jun 90 13:30:22 GMT
References: <1990Jun26.175953.16763@uncecs.edu>
Sender: usenet@linus.mitre.org
Reply-To: dsr@luke.mitre.org (Douglas S. Rand)
Distribution: comp
Lines: 41

In article <1990Jun26.175953.16763@uncecs.edu>, harris@uncecs.edu (Mark
Harris) writes:
|>
|>Help!  I'm fairly new to C++, and I'm running into unexpected 
|>behavior from my new Borland compiler.  I thought that the value 
|>of 'this' was supposed to be zero at the start of an object's 
|>constructor if the object is being created on the heap.  I 
|>expected output 
|>  nonzero
|>  zero
|>in the program below, but I get
|>  nonzero
|>  nonzero
|>What gives?
|>
|>#include <stdio.h>
|>struct test{
|>  int a;
|>  test(){
|>    if(this==0)puts("zero");
|>    else puts("nonzero");
|>  }
|>};
|>
|>void main()
|>{
|>  test t;
|>  test *x=new test;
|>}
|>

`this' can never be zero in a constructor.  It's just been malloc'd or
allocated on the heap
and already has a distinct address.  If this wasn't true you couldn't
assigned values to 
the instance variables,  you'd have a zero pointer dereference.
                                  
Douglas S. Rand 
Internet:   <dsrand@mitre.org>
Snail:	    MITRE, Burlington Road, Bedford, MA 
Disclaimer: MITRE might agree with me - then again...