assert(): a "tutorial"

Tim McDaniel mcdaniel at amara.uucp
Fri Apr 13 09:23:24 AEST 1990

u096000 at (Roger A. Cole) wrote a good article about assert().
assert() doesn't get enough respect: it can be a BIG lifesaver, and a
significant help in documentation!  (As Roger pointed out.)

In the article, he gave a definition
   #ifdef DEBUG
   #   define assert(expr) ((void)((expr) || assertFail(__FILE__, __LINE__)))
   #    define assert(expr)

However, one of the examples given was
   assert(strlen(name) < 20), printf("%s\n", name);
If DEBUG were not defined, this would expand to
   , printf("%s\n", name);
which is not syntactically valid.  Making the second choice be
   #    define assert(expr)	((void) 0)
would suffice.

(Personally, I prefer to print the expression that failed, which could
help if I wrote two assertions on one line.  I also like to give a
constant string as an explanation argument, for non-obvious checks:
	assert(frobozz(queue) != xyzzy(reg), "pipeline lost a datum");
If, instead, you put an explanatory comment there, the effect would be
more-or-less identical, but requiring a second argument forces the
programmer to provide an explanation.  Personal style.)

Tim McDaniel
Applied Dynamics International, Ann Arbor, MI
Internet: mcdaniel%amara.uucp at
UUCP: {uunet,sharkey}!amara!mcdaniel

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