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From: ballou@brahms (Kenneth R. Ballou)
Newsgroups: comp.lang.c
Subject: Re: incrementing after a cast
Message-ID: <518@cartan.Berkeley.EDU>
Date: Mon, 8-Dec-86 15:22:07 EST
Article-I.D.: cartan.518
Posted: Mon Dec  8 15:22:07 1986
Date-Received: Mon, 8-Dec-86 21:46:13 EST
References: <349@apple.UUCP> <7376@utzoo.UUCP> <1746@batcomputer.tn.cornell.edu>
Sender: daemon@cartan.Berkeley.EDU
Reply-To: ballou@brahms (Kenneth R. Ballou)
Organization: Math Dept. UC Berkeley
Lines: 52
Keywords: cast, increment, postincrement

In article <1762@batcomputer.tn.cornell.edu> braner@batcomputer.UUCP (braner) writes:
>[]
>
>In article <1746@batcomputer.tn.cornell.edu>  I wrote:
>>If  ((sometype *)pointer)++ is not legal, then the law should be changed!
>>(since the pointer-type cast is legal, why can't you use the resulting
>>pointer like any pointer of sometype?)
>
>In article <491@cartan.Berkeley.EDU> ballou@brahms (Kenneth R. Ballou) wrote:
>>You can use the *VALUE* of the pointer in exactly the same way as you could
>>use the *VALUE* of any other pointer. ...the pre/post-increment
>>operator requires one of those mythical beasts, an l-value.  Likewise, you
>>could not apply pre/post-increment to the result of a function call.
>
>- When you cast a pointer to a pointer of another type, you are telling
>the compiler to use it

Thank you.  Exactly what is *it*?  *It* is the VALUE of the pointer.  This is
what is called "r-value" in Kernighan and Ritchie, and in Harbison and Steele.

> 			  differently (e.g. to read 2 instead of 1 byte
>from memory when dereferencing).  Since the compiler is aware of the cast
>in that sense, it COULD increment it according to the new type's size!

WRONG.  Do you have any clue what the difference is between "l-value" and
"r-value"?  A CAST DOES NOT PRODUCE AN L-VALUE!  I quote from Kernighan and
Ritchie, page 187:

	    An expression preceded by the parenthesized name of a data
	type causes conversion of the value of the expression to the
	named type.  This construction is called a 'cast.'

Then again, here is another quotation from Harbison and Steele, page 152:

	    The cast causes the operand value to be converted to the
	type named within the parentheses.  Any permissible conversion
	may be invoked by a cast expression.  The result is not an
	lvalue.

>I see no TECHNICAL obstacle here, only "legal" morass...

	May I then suggest you look again.  It seems to me that if something
is not an l-value, the compiler is not to use it as an l-value.

Perhaps I could put in a good word with Santa Claus and ask him to leave
you a copy of K&R, or maybe even Harbison & Steele?

--------
Kenneth R. Ballou		ARPA: ballou@brahms.berkeley.edu
Department of Mathematics	UUCP: ...!ucbvax!brahms!ballou
University of California
Berkeley, California  94720