Path: utzoo!attcan!uunet!world!decwrl!wuarchive!emory!gatech!uflorida!travis!tom
From: tom@ssd.csd.harris.com (Tom Horsley)
Newsgroups: comp.lang.c++
Subject: Constructor called with reference to self?
Message-ID: <TOM.90Nov2072213@hcx2.ssd.csd.harris.com>
Date: 2 Nov 90 12:22:13 GMT
Sender: news@travis.csd.harris.com
Organization: Harris Computer Systems Division
Lines: 46

Given a class that looks like:

class someclass {
public:
   someclass(someclass& arg);
   ...
}

Is it ever possible with a correctly functioning C++ compiler for your
program to reach the body of the if statement in the following copy
constructor:

someclass::someclass(someclass& arg)
{
   if (this == &arg) {
      // Can I get here?
   }
}

In other words can a copy constructor ever be called with a reference to
self as its argument?

I hope the answer is 'NO' since all the C++ books emphasize that
constructors only get raw bits and it is their job to initialize those bits.
So if '*this' is raw bits and 'arg' is a reference to an already existing
class, how can they be located at the same point in memory?

I spent several hours last night determining that this is actually happening
in a Turbo C++ program I wrote in which I have overloaded a lot of
arithmetic operators and have written some complex expressions involving
these overloaded operators. When I modified my constructors to detect this
case and avoid clobbering the information it is about to copy, the program
started working flawlessly (it worked on two different g++ implementations
before I tried it on Turbo C++).

Does this look as much like a bug to everyone else as it does to me, or have
I possibly encountered one of those subtle gotcha's that always seem to
attract programmers who are just starting to use a new language?
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