Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!rpi!crdgw1!crdos1!davidsen
From: davidsen@crdos1.crd.ge.COM (Wm E Davidsen Jr)
Newsgroups: comp.lang.c
Subject: Re: sizeof a struc field
Message-ID: <1469@crdos1.crd.ge.COM>
Date: 25 Oct 89 18:03:11 GMT
References: <7710@microsoft.UUCP> <680011@hpmwjaa.HP.COM> <11404@smoke.BRL.MIL> <10767@csli.Stanford.EDU>
Reply-To: davidsen@crdos1.UUCP (bill davidsen)
Organization: GE Corp R&D Center
Lines: 32

In article <10767@csli.Stanford.EDU>, poser@csli.Stanford.EDU (Bill Poser) writes:
|  I am curious as to when someone would want to take sizeof(((T *) 0)->member).
|  If you need to know the size of a structure member, you can always consult
|  the structure definition, discover that member is, say, of type "int",
|  and write sizeof(int). If you want to allow for future changes in the
|  member's type, use the preprocessor and make both the definition and
|  the argument to sizeof macros, e.g.

  This is just not practical unless you control the header file. There
are times when a field in a structure may not be the same type
(portably). While it's nice to dream of compliant C, you don't run real
software on it. A field might be short on a VAX, int on a PC, and
fubar_t on an ANSI compiler. Therefore, to write code which is maximally
portable you must take the sizeof the field.

  No one argued that you can't get the sizeof a field without using a
cast zero with -> to do it, just that it was a useful way to do it, that
most current compilers support it, that no one has proposed a simple
alternative which doesn't increase program size, and I have argued that
that I still think it's legal under the standard.

  For all the people who proposed using an extern and not defining it,
that generates a linker error on some systems and is not portable. A
conforming implementation does not have to ignore extern declarations
which are not used.


-- 
bill davidsen	(davidsen@crdos1.crd.GE.COM -or- uunet!crdgw1!crdos1!davidsen)
"The world is filled with fools. They blindly follow their so-called
'reason' in the face of the church and common sense. Any fool can see
that the world is flat!" - anon