Path: utzoo!utgpu!water!watmath!clyde!burl!codas!killer!jfh
From: jfh@killer.UUCP (The Beach Bum)
Newsgroups: comp.lang.c
Subject: Re: lvalues (was Re: Question about type casting)
Summary: Slightly more complicated that it really is.
Message-ID: <2975@killer.UUCP>
Date: 18 Jan 88 06:25:04 GMT
References: <546@xyzzy.UUCP> <5080012@hpfcdc.HP.COM> <10227@mimsy.UUCP>
Organization: Big "D" Home for Wayward Hackers
Lines: 59

In article <10227@mimsy.UUCP>, chris@mimsy.UUCP (Chris Torek) writes:
> In some article somewhere, someone writes:
> >>I believe that Standard C does not allow type casting on the left of the 
> >>= sign.
> 
> In article <5080012@hpfcdc.HP.COM> boemker@hpfcdc.HP.COM (Tim Boemker) writes:
> >How about this:
> >
> >int i; char *c;
> >c = (char *) &i;
> >* (int *) c = 0;
> 
> It is not portable, but it is legal.
 
I believe it used to be portable.  Now that X3J11 has mangled the language,
it isn't guaranteed to be portable.  If I recall correctly, a pointer to
any object can _portably_ be converted to a pointer to a smaller object and
back again.  Thus,

	int i; char * cp;
	cp = (char *) &i;
	* (int *) cp = 0;

is portable, but

	int *ip; char c;
	ip = (int *) &c;
	* (char *) ip = '0';

is not.

> The result of a cast is an `rvalue' (R for Right: or in other words,
> the kind of thing one finds on the right hand side of an assignment).
> Assignments may be made only to `lvalues' (L for Left: the kind of
> thing found on the left hand side of an assignment).  The C indirection
> operator `*' takes an rvalue and yeilds an lvalue, hence the result
> of `* (int *) c' is an lvalue.
> -- 
> In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)

Chris, that's some good voodoo.  A cast takes an object of whatever
type it happens to be, and whatever state of (l|r)value-ness and changes
it's type, but not it's (l|r)value-ness.  The difference between lvalue
expressions and rvalue expressions is more obvious in a language such
as BLISS with all those silly dots. . . . . .

The only restriction on what can be on the left hand side of an `=' is
having an address and a type, or being finaglable into having both.  I
can cast an obvious rvalue like `2' into an int * with ((int *) 2).
This thing can now appear on either side -

	*((short *) 2) = *((short *) 4);

- John.
-- 
John F. Haugh II                  SNAIL:  HECI Exploration Co. Inc.
UUCP: ...!ihnp4!killer!jfh                11910 Greenville Ave, Suite 600
"Don't Have an Oil Well? ...              Dallas, TX. 75243
 ... Then Buy One!"                       (214) 231-0993 Ext 260