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From: kanner@apple.UUCP
Newsgroups: comp.lang.c++
Subject: Explanation desired
Message-ID: <807@apple.UUCP>
Date: Wed, 20-May-87 21:43:41 EDT
Article-I.D.: apple.807
Posted: Wed May 20 21:43:41 1987
Date-Received: Sat, 23-May-87 01:18:34 EDT
Organization: Apple Computer, Inc., Cupertino, USA
Lines: 45
Keywords: virtual

I have been very puzzled by some of the C code produced by cfront when
declaring an object for a class that has virtual functions.  Here is
part of an example:

C++ source:

struct base{
	virtual void f1();
	virtual void f2();
	virtual void f3();
};

struct d1 : base {
	virtual void f1();
	virtual void f2();
};

struct d11 : d1 {
	virtual void f1();
	virtual void f2();
};

d11 od11;

...


Now the C code produced by "d1 od11;" follows; I have removed 17
levels of redundant parentheses, include 14 consecutive left-hand ones
at the start, and I have simplified the identifier _au1_od11 to od11
for readability:

(struct base *) (struct d1 *) &od11->_base__vptr = base__vtbl,
(struct base *) (struct d1 *) &od11,
(struct d1 *) &d11->_base__vptr = d1__vtbl,
(struct d1 *) &od11;
&od11->_base__vptr = d11__vtbl, // This line appears to be valid
&od11;

As far as I can tell, only the line I have commented appears to do
anything correct and useful.  Can anyone tell me what is going on?
-- 
Herb Kanner
Apple Computer, Inc.
{idi, ios, nsc}!apple!kanner