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From: wfp@dasys1.UUCP (William Phillips)
Newsgroups: comp.lang.c++
Subject: Interesting "Feature" of C++
Message-ID: <1718@dasys1.UUCP>
Date: Tue, 20-Oct-87 01:46:07 EDT
Article-I.D.: dasys1.1718
Posted: Tue Oct 20 01:46:07 1987
Date-Received: Fri, 23-Oct-87 01:56:59 EDT
Organization: The Big Electric Cat
Lines: 72
Keywords: virtual functions, assigning to this

I have discovered an interesting "feature" of C++ (undocumented, as far as I
know) which takes effect when using virtual functions and assignments
to  this .

Briefly, given a base class and derived class(es) containing virtual function(s),
if your program makes an assignment to  this , you may be in trouble.
Example:

struct base {
	.
	.
	base *something();		// defined elsewhere
	base *something_else();
	virtual void whatever();
	.
	.
	.
}

struct derived : base {
	.
	.
	.
	void whatever();
	.
	.
	.
}

	.
	.
	.
	this = something();
	if (this == NULL) {
		this = something_else();
	}
	.
	.
	.

A fragment similar to the above, if the result of the assignment
	this = something();
is in fact null, will (on a Silicon Graphics Iris workstation)
cause a segmentation violation at runtime.  If you explicitly assign
	this = NULL;	(or this = 0;)
the problem will not occur.  The reason is that the C code generated by
cfront in the first case does more than assigning the result of a
function call; it also sets a pointer to an array of pointers to the
virtual functions.  It does not generate code to check the
value of  this  before attempting to set the pointer, however.  Thus, if this
is NULL, the program attempts to set the function pointer to some bizarre
value, and crashes.  An explicit assignment of 0 to  this  does not generate
code for changing the function pointer, and so causes no problem.  The
workaround is simple, but annoying (untidy!):

	base *that = something();
	if (that == NULL) {
		that = something_else();
	}
	this = that;
	.
	.
	.
or something like that. 

Any comments?


-- 
William Phillips                 {allegra,philabs,cmcl2}!phri\
Big Electric Cat Public Unix           {bellcore,cmcl2}!cucard!dasys1!wfp
New York, NY, USA                (-: Just say "NO" to OS/2! :-)