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From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: comp.lang.c++
Subject: Re: value of 'this' in constructor
Message-ID: <11206@alice.UUCP>
Date: 17 Aug 90 13:08:19 GMT
References: <12680@hydra.gatech.EDU>
Distribution: na
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 34

In article <12680@hydra.gatech.EDU>, gt3930b@prism.gatech.EDU (NICHOLS,STEVEN MALCOLM) writes:

> 	Consider the following piece of code.  From p. 284 of "Using
> C++", and also p. 164 of "The C++ Programming Language", I am lead to 
> believe that the value of the pointer 'this', on entry to a constructor,
> will be 0 if the instance of the class has been dynamically allocated.

If you are using the now-deprecated feature of assigning to `this,'
and you are using a compiler that supports the feature, then
`this' will be 0 on entry to your constructor -- BUT ONLY IF THE
CONSTRUCTOR ASSIGNS TO `this' (believe it or not!).

That is, the presence of an assignment to `this' in a constructor
makes that constructor magic: not only is `this' then zero on entry
to the constructor but you are obliged to assign some value to it
on *every* path through the constructor.  The following is typical:

	Foo::Foo() {
		if (this == 0) {
			this = /* some appropriate pointer */
		} else {
			this = this;	// believe it or not
		}

		// now you can deal with members of the object
	}

The idea is that the act of assigning to `this' triggers initialization
of base classes, and so on.

You see why the feature is deprecated?
-- 
				--Andrew Koenig
				  ark@europa.att.com