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From: crowl@cs.rochester.edu (Lawrence Crowl)
Newsgroups: comp.lang.c
Subject: Re: typeof isn't enough to define swap correctly
Message-ID: <1880@sol.ARPA>
Date: Fri, 4-Sep-87 17:52:50 EDT
Article-I.D.: sol.1880
Posted: Fri Sep  4 17:52:50 1987
Date-Received: Sat, 5-Sep-87 19:57:20 EDT
References: <557@rocky.STANFORD.EDU>
Reply-To: crowl@cs.rochester.edu (Lawrence Crowl)
Organization: U of Rochester, CS Dept, Rochester, NY
Lines: 38

In article <557@rocky.STANFORD.EDU> andy@rocky.UUCP (Andy Freeman) writes:
]In article <1354@dataio.Data-IO.COM>, bright@dataio.Data-IO.COM (Walter Bright) writes:
]> ... One thing I want, which the committee always seems to poo-poo, is a
]> typeof. Having a typeof would make creation of 'generic' routines possible:
]
]> 	#define swap(a,b)	{ typeof(a) tmp; tmp = a; a = b; b = tmp; }
]
]Even given typeof, this definition of swap is buggy.  (It has at least
]two bugs.)  I've never figured out how to define a C swap macro that
]works - I'm willing to believe it is impossible.
]
]Anyone have a use for typeof that isn't doesn't have the major bug that
]the swap macro defined above has?

The following swap macro is correct.  It relies on the pointer features of C
to evaluate its arguments exactly once.

#define swap(a,b) { \
    typeof(a) t,*p1,*p2; /* temporary and pointers to sources */ \
    p1 = &(a); p2 = &(b); /* evaluate arguments exactly once */ \
    t = *p1; *p1 = *p2; *p2 = t; /* do actual swap */ \
}

The following example works.  (Well, I substituted typeof(a) with int.)

int a[] = { 0, 1, 2, 3 } ; int b[] = { 0, 1, 2, 3 } ;
int i = 0 ; int j = 1 ;

swap(a[i++],b[j++]);

a == { 1, 1, 2, 3 }; b == { 0, 0, 2, 3 };
i == 1; j == 2;

One problem still remains.  What if typeof(a) != typeof(b)?
-- 
  Lawrence Crowl		716-275-9499	University of Rochester
		     crowl@cs.rochester.arpa	Computer Science Department
 ...!{allegra,decvax,seismo}!rochester!crowl	Rochester, New York,  14627