Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!lll-winken!uunet!ncrlnk!ncr-sd!hp-sdd!hplabs!hp-ses!hpcuhb!hpcllla!hpclisp!hpclwjm!walter
From: walter@hpclwjm.HP.COM (Walter Murray)
Newsgroups: comp.lang.c
Subject: Re: The Dangers of sizeof
Message-ID: <660031@hpclwjm.HP.COM>
Date: 13 Apr 89 19:10:23 GMT
References: <105@servio.UUCP>
Organization: Hewlett-Packard Calif. Language Lab
Lines: 35

> Trick question:  what is the value of:
> 
> sizeof (char) - 1
> 
> Answer:  1

I know that many others will point out that this is wrong.  But wait:
there *IS* reason for confusion. 

If you just look at page 49 of K&R (page 53 in the 2nd edition), 
you will think that sizeof, (type), and unary - are operators with
the same precedence, and that they associate right-to-left.  By that,
the answer *SHOULD* be 1, shouldn't it?  

On page 188 (first edition) you find "the rest of the story":  They tell
you that sizeof(type) "is to be taken as a unit".  In other words,
the sizeof operator actually has higher precedence than a cast.  In the
second edition, I can't find any place where that is spelled out in so
many words, though it does follow from the syntax rules that are given.
(A "cast-expression" is now distinct from a "unary-expression".)  

I hope the precedence table gets updated in some future edition.  Another
problem in the table (troublesome only for novices) is that operators
like "-" appear twice, once as unary and once as binary.  But now I'm
drifting.

Back to the original problem, Harbison & Steele also comment on it
(2nd edition, page 156).  They see it as a syntactic ambiguity which
is resolved arbitrarily.

The pANS resolves it, of course, by having unary operators (like
sizeof) at a higher precedence than cast operators.

Walter Murray
-------------