Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!haven!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.lang.c
Subject: Re: Do you have to cast void pointers when dereferencing them?
Keywords: (With ANSI C)
Message-ID: <15012@mimsy.UUCP>
Date: 15 Dec 88 01:02:23 GMT
References: <2414@ssc-vax.UUCP>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 25

In article <2414@ssc-vax.UUCP> dmg@ssc-vax.UUCP (David Geary) writes:
[edited]
>struct junk { int   x,y; };
>    struct junk J;
>    void  *p;
>    p = &J;
>    ... p->x ...

The assignment `p = &J' (uncast) is legal dpANS C.  The reference
`p->x' is not.  To see why, consider this fragment:

	struct foople { char alpha, omega; } f;
	struct gluxet { int rho, omega; } g;
	void *p;
	if (flipcoin() == HEADS) p = &f; else p = &g;
	printf("%d\n", p->omega);

Which omega should be used?

(I used `if ... p = &f; else p = &g;' instead of `p = ... ? &f : &g;'
to avoid questions about mixing ?: pointer types, which, when last
I checked, seemed rather muddled.  The sensible approach is to make
`... ? &f : &g' illegal.)
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris