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From: rh@smds.UUCP (Richard Harter)
Newsgroups: comp.lang.c
Subject: Re: The nonexistent operator (along = v. == lines)
Summary: This may be what you want.
Keywords: xor
Message-ID: <372@smds.UUCP>
Date: 5 Apr 91 07:33:52 GMT
References: <156@revcan.UUCP> <157@revcan.UUCP>
Organization: SMDS Inc., Concord, MA
Lines: 29

In article <157@revcan.UUCP>, darren@revcan.UUCP (Darren Morbey) writes:

> What I did require was an operator, macro, or function that treated
> its operands as "zero" or "non-zero" as && and || do rather than
> the bitwise & | ^.  I also would like some guarantee that both
> operands were evaluated *once* *and* *only* *once* (O&OO)...

Then I believe that what you want is

#define XOR(a,b) (  !(a)  ?  (!!(b))  :  (!(b))  )

which returns 1 if exactly one of the two arguments is 0 and 0 otherwise,
with both arguments being evaluated exactly once.  As far as I can see this
is the only way to meet your requirements.  I am not at all sure if it is
possible to write macros for AND and OR that meet your requirements.  The
problem I see is this:

#define AND(a,b)  ( !(a) ? ((b) && 0) : !!(b)  )

appears to do the trick, modulo typoes.  However it occurs to me that a
"clever" optimizing compiler would recognize that ((b) && 0 ) is always
false and bypass the evaluation of b.  Perhaps the language lawyers can
tell us if the language specifications *require* that b be evaluated.

-- 
Richard Harter, Software Maintenance and Development Systems, Inc.
Net address: jjmhome!smds!rh Phone: 508-369-7398 
US Mail: SMDS Inc., PO Box 555, Concord MA 01742
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