Re: Inheritance and offsetof

From:

=?ISO-8859-1?Q?Marcel_M=FCller?= <news.5.maazl@spamgourmet.org>

Newsgroups:

comp.lang.c++

Date:

Tue, 22 Sep 2009 00:16:25 +0200

Message-ID:

<4ab7fb39$0$31864$9b4e6d93@newsspool3.arcor-online.net>

Francesco S. Carta wrote:

Also, I think that either there is a problem in your structoffsetof
macro or in my understanding of your purposes (likely so) because it
equates to zero while I would have expected it to equate to "sizeof
(A)".

Zero is correct for the most implementations, since A is the first base
class of B and both are non-polymorphic. But this macro does not raise
the warning. Calling offsetof is the problem.

About the other macro, "offsetof(B, m3) == (size_t(&b.m3) - size_t
(&b))" on my system,

The offset is needed relative to the start of A because A* is passed to
the C library.

hence I cannot see the need for that macro, which
does castings all over the place.

If you already have an instance of your type, you are right, but
otherwise you have to make your own pseudo-instance. And that is exactly
the way the macros offsetof and my structoffsetof work with all that casts.

Furthermore when dealing with an instance I would prefer to cast to
char*: (char*)&b.m3 - (char*)&(A&)b

Marcel