#define NDEBUG disables
#define NS_BLOCK_ASSERTIONS disables
NSAssert(). One kind of assert being disabled does not mean the other kind is. Nether kind of assertion is disabled by default when building in “Release” mode — you must be sure to disable them yourself. Be careful, a user will not appreciate the difference between a bug and a failed assertion — both are a crash.
assert() immediately stops your program with a
SIGABORT signal. You can’t miss a failed
assert(), its behavior is predictable, and it fails fast (link is a PDF).
But a failed
NSAssert() behaves unpredictably. Sometimes it stops program execution, sometimes it does not and leaves the program in a strange inconsistent state. I have entirely missed
NSAssert()s failing, because I did not look at the console.
if(!expr) NSLog(); would have been a better choice in the those cases, because at least I would have known how it behaved.
Assertion macros, such as NSAssert and NSCAssert … pass a string to an
NSAssertionHandlerobject describing the failure. Each thread has its own
NSAssertionHandlerobject. When invoked, an assertion handler prints an error message that includes the method and class (or function) containing the assertion and raises an
Complicated! The complexity means it is possible to customize what happens when an
NSAssert() fails. That sounds cool, but I’ve never heard of someone needing to actually do that.
If a framework catches
NSInternalInconsistencyExceptions, then your program will keep right on running after a failed
NSAssert(). I have had this happen to me several times. I apologize for not having taken the time to investigate what frameworks were catching what.
Apple could change what catches what with any software update.
Variability and complexity are the last things you want while debugging. There’s no reason to invite it them by using
NSAssert() is not guaranteed to stop your program, it can not be relied on to guard against data corruption, or anything worse then a predictable crash.
UPDATE 2009-06-01: You can annotate
assert(), so it prints an explanation like
&&ing in a string after the condition. For example
assert(i < j) is a lot more useful with an explanation:
assert(i < j && "unexpected cycle in graph") — on failure it prints
Assertion failed: (i < j && “unexpected cycle in graph”), function WillHalt(), file /path/to/code.m, line 30.