Question

I’m trying to understand exceptions a little more. I’ve installed in my AppDelegate an uncaught exception handler:

// in applicationDidFinishLaunching:
NSSetUncaughtExceptionHandler(&HandleExceptions);
...
void HandleExceptions(NSException *exception) {
    // Save application data on crash
    [appDelegate saveData];
}

So I’m not entirely sure how this works. Say I access an array out of bounds, which throws an exception. Why does this method not get called? Or say there is an assertion exception thrown after inserting or removing rows from a table – how can I save data before the app crashes in those situations?

In general, when the system throws an exception, how can I have the chance to save data before the app crashes? Or does it not work like that? Is the uncaught exception handler only for exceptions that I throw that are not enwrapped in a @catch or @try?

Answer 1

Perhaps ‘appDelegate’ is nil when your method is called. I just added this to a viewController:

void HandleExceptions(NSException *exception) {
    // Save application data on crash
    NSLog(@"YIKES");
}

then did this:

NSArray *foo = @[ @1, @2, @3 ];
NSSetUncaughtExceptionHandler(&HandleExceptions);
NSLog(@"%@", [foo objectAtIndex:3]);

and got this:

2012-09-06 13:51:17.474 Searcher[64798:f803] YIKES
2012-09-06 13:51:17.475 Searcher[64798:f803] *** Terminating app due to uncaught exception 'NSRangeException', reason: '*** -[__NSArrayI objectAtIndex:]: index 3 beyond bounds [0 .. 2]'
*** First throw call stack:
(0x67f022 0x1749cd6 0x66b644 0x324d 0xbb8a1e 0xb17401 0xb17670 0xb17836 0xc9addd8 0xb1e72a 0x2fcd 0xaef386 0xaf0274 0xaff183 0xaffc38 0xaf3634 0x2260ef5 0x653195 0x5b7ff2 0x5b68da 0x5b5d84 0x5b5c9b 0xaefc65 0xaf1626 0x2dfc 0x2d95