Path: utzoo!mnetor!uunet!husc6!cmcl2!brl-adm!brl-smoke!gwyn
From: gwyn@brl-smoke.ARPA (Doug Gwyn )
Newsgroups: comp.lang.c
Subject: Re: lvalues (was Re: Question about type casting)
Message-ID: <7131@brl-smoke.ARPA>
Date: 19 Jan 88 14:57:42 GMT
References: <546@xyzzy.UUCP> <5080012@hpfcdc.HP.COM> <10227@mimsy.UUCP> <2975@killer.UUCP>
Reply-To: gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 27

In article <2975@killer.UUCP> jfh@killer.UUCP (The Beach Bum) writes:
>In article <10227@mimsy.UUCP>, chris@mimsy.UUCP (Chris Torek) writes:
>> 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.
>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.

If you don't know what you're talking about, you should shut up!

Chris is right (as usual) -- a cast produces a non-lvalue (which you
may call an `rvalue' if you wish), and it is only the result of
applying a * operator that is an lvalue as required for the target
of the assignment.

A cast performs a simultaneous conversion and virtual assignment of
its operand into an unnamed temporary register, in effect.  There
is no way that the result can be conceived of as being addressable.

>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.

Wrong -- it must be a modifiable lvalue.