Path: utzoo!attcan!uunet!cs.utexas.edu!wuarchive!decwrl!wsl.dec.com!haynes
From: haynes@wsl.dec.com (Charles Haynes)
Newsgroups: comp.lang.c++
Subject: Re: calculate member byte offset
Message-ID: <1908@bacchus.dec.com>
Date: 8 Oct 89 13:10:00 GMT
References: <6590284@hplsla.HP.COM> <1989Oct5.193826.24178@polyslo.CalPoly.EDU>
Sender: news@decwrl.dec.com
Reply-To: haynes@wsl.dec.com (Charles Haynes)
Organization: DEC Western Software Lab
Lines: 42

The (relatively gross) hack we used in the X Intrinsics is:

#define XtOffset(type,field)    ((unsigned int)&(((type)NULL)->field))

This has a different set of problems from your example, but works on most
architectures/compilers. Some "C"'s provide an offset builtin that you 
could use. As for the problems with your example:

> class foo
> {
> public:
>   int grr;
>   int header;		//does header stuff really need to be public?
>   void alloc();
> };
> 
> void foo::alloc()
> {
>   cout << (&(this->header)) - this << "\n";   // bomb! 2.0 complains 
> // that this is of type foo*, while &(this->header) is of type int*.
> // Maybe you can get away with coercing them both to type char*, but
> // not guaranteed to work with all compilers

That's right! You are subtracting things of different type. *I* wouldn't
expect it to work in general. This is where I'd use the XtOffset macro.

> }
> 
> main()
> {
>   ((foo*) 0) -> alloc();   // bomb?  -- possible on a machine that
> 		  	   // disallows dereferencing a null pointer

This should be possible on ANY machine. You aren't actually dereferencing
anything, you're just selecting a member function and passsing "this" to
it. "alloc" had better be ready to get a NULL "this" though.

> }

Hope this is helpful.

	-- Charles