Path: utzoo!attcan!uunet!mcvax!hp4nl!botter!star.cs.vu.nl!maart
From: maart@cs.vu.nl (Maarten Litmaath)
Newsgroups: comp.std.c
Subject: Re^2: struct comparison
Message-ID: <2878@kappl.cs.vu.nl>
Date: 15 Jul 89 00:30:41 GMT
References: <2874@solo3.cs.vu.nl> <1989Jul14.155312.2063@utzoo.uucp>
Organization: V.U. Informatica, Amsterdam, the Netherlands
Lines: 56

henry@utzoo.uucp (Henry Spencer) writes:
\In article <2874@solo3.cs.vu.nl> maart@cs.vu.nl (Maarten Litmaath) writes:
\>Why does the PROGRAMMER have to go through all that trouble?
\>I just want to say:
\>
\>	puts(mork == mindy ? "equal" : "unequal");
\>
\>Is the ANSI committee trying to tell us the compiler cannot transform the
\>equality test into the correct member-by-member comparison code?
\
\Yes.  Think about unions.  Or pointers (do you compare the pointers or
\what they point at?).  The compiler just doesn't have enough information.

I thought of those as well:
1)	pointers - They should be compared themselves; if they don't point to
	the same location, they're unequal by definition; if the data they
	point at are equal, that's mere coincidence.
2)	unions - Compare with the following example:

		struct	foo {
				char	bar[10];
			} x, y;
		
		(void) strcpy(x.bar, "123456789");
		(void) strcpy(y.bar, "987654321");
		(void) strcpy(x.bar, "zork");
		(void) strcpy(y.bar, "zork");

		puts(x == y ? "yes" : "no");

	I say the output should be "no", because all 10 bytes in `bar' must
	be compared. (The array needn't be a string at all! I might have used
	it to store small integers.)
	If you want to get "yes", you have to make sure there's no garbage
	at the end of `bar', i.e. you must clear the unused part explicitly.
	Unions can be dealt with likewise.

I KNOW these two specifications might cause some surprises in actual code,
but:
1)	Compare with this little gem to confuse Pascal programmers:

		if ("string" != "string")
			puts("how weird!");

2)	The following example shows how useful struct comparison could be:

		struct	complex {
				int	real;
				int	imag;
			} z, w;

		...
		if (z == w) ...
-- 
   "... a lap-top Cray-2 with builtin    |Maarten Litmaath @ VU Amsterdam:
cold fusion power supply"  (Colin Dente) |maart@cs.vu.nl, mcvax!botter!maart