Path: utzoo!attcan!uunet!microsoft!jimad
From: jimad@microsoft.UUCP (Jim ADCOCK)
Newsgroups: comp.lang.c++
Subject: Re: testing this==0
Keywords: this
Message-ID: <55488@microsoft.UUCP>
Date: 27 Jun 90 17:59:00 GMT
References: <1990Jun26.175953.16763@uncecs.edu>
Reply-To: jimad@microsoft.UUCP (Jim ADCOCK)
Distribution: comp
Organization: Microsoft Corp., Redmond WA
Lines: 38

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.  

While the (this==0) test in mentioned in passing in Stroustrup's 1986
text, I believe it should be considered part of the assignment-to-this
anachronism.  In fact, even the compilers I have seen that support the
assignment-to-this anachronism don't support the (this==0) test for
objects on the heap.  Only the very first C++ compiler I tried some
years ago ever actually support the (this==0) test.

The present definition on how things are suppose to work is as follows:
a) don't assign to this, if you need to modify memory allocation behavior,
overload operator new. b) if operator new ever returns zero, it is guaranteed
that initialization will not be performed -- the constructor body will not even
be executed, so in the following constructor:

class mything
{
public:
	mything() { if (this==0) printf("Non-conforming compiler!\n"); }
};

the printf should never occur, even if operator new fails to successfully
allocate memory.

[see E&S for the detailss]

So the question becomes: is there a standard, portable way to test whether
allocation is being done on the stack, heap, or wherever?  Answer: nope!
There are lots of non-standard, non-portable ways to test this, however.
Possibly the most portable thing to do is to test if this is in a region
reasonably considered stack, since stack is typically contiguous [heap need
not be.]  Could still cause you troubles in a lightweight threaded application,
however.