Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!hsi!stpstn!cox
From: cox@stpstn.UUCP (Brad Cox)
Newsgroups: comp.software-eng
Subject: Re: "Program Proving"
Message-ID: <4389@stpstn.UUCP>
Date: 21 Mar 90 22:26:26 GMT
References: <1990Mar17.063012.24979@agate.berkeley.edu> <HUGHES.90Mar19084416@tempest.Berkeley.EDU>
Reply-To: cox@stpstn.UUCP (Brad Cox)
Organization: Stepstone
Lines: 38

Apparently you haven't heard that C (and its derivatives, including
Objective-C and undoubtedly C++ as well) supports assertion checking.
It's called assert(). You will undoubtedly find it in /usr/include/assert.h
on your system, or thereabouts.  You use it like this

	#include "assert.h"
	assert(YES)
	a = 1;
	assert(a == 1);

The macro generates
	if (!(1)) {
		fprintf(stderr, "assertion failure in file %s line %d\n",
			_FILE_, _LINE_);
		exit(1);
	}
	a = 1;
	if (!(a == 1)) {
		fprintf(stderr, "assertion failure in file %s line %d\n",
			_FILE_, _LINE_);
		exit(1);
	}

But if you set the macro variable, NDEBUG, the assert()s vanish at
compile time, so they can be left in the source forever as documentation.

By the way, I recommend that the macro be modified to generate as follows,
for reasons that should become obvious once you start using it a lot.

	if (!(a == 1)) assertionFailure(_FILE_, _LINE_);

This has proven so useful for C code in general, and for polymorphic
object-oriented Objective-C code in particular, that assertion-checking 
has become a habit, something that we take for granted that any professional
programmer would be using as a matter of routine.

Wonder why the existence of this macro comes as a surprise to so many. 
I've even heard assertion checking used as a selling point for Eiffel!