Chapter 9, Exercise 6

[nate]

I can't seem to get a division by zero exception to be trapped by a simple try/catch block.  I tried the following code:

Code: (Objective-C)

    @try {
        accumulator /= value;
    }
    @catch ( NSException* exception ) {
        NSLog( @"Caught %@: %@", [exception name], [exception reason] );
    }

Which didn't do much (the division by zero produced -inf, but no exception was raised). 

So I changed the program to use integers instead of floating point numbers, after which the program would exit prematurely with the message "Floating point exception."  Which is odd for two reasons: 1) it is all integer math now, so why a floating point exception and 2) why wasn't the exception propagated up from the system into the Objective-C try/catch block?

Ultimately, I read the Xcode documentation section on throwing exceptions and found an example of how to throw one, so I solved the Exercise as follows (which works for both floating point and integer math):

Code: (Objective-C)

    @try {
        if ( value == 0 ) {
            NSException *e = [NSException exceptionWithName: @"Floating point exception"
                                          reason: @"Division by zero" userInfo: nil];
            @throw e;
        }
        accumulator /= value;
    }
    @catch ( NSException* exception ) {
        NSLog( @"Caught %@: %@", [exception name], [exception reason] );
    }

Perhaps I've missed something and this is indeed the intended solution for the exercise? 

If not, is there something I'm doing wrong?  It sure seems a lot more convenient to write the simpler try/catch and count on the system exception climbing up the exception chain instead of using the manual check in my solution.

[mdeh]

Hi Nate,
Not to worry...that exercise is deleted. Go to the errata and download the errata for the 2nd printing. I was trying to find the discussion about this, but that seems to have gone too...but it , as you have found it, is not possible to do given the current OS and the exercise as written.

Here's a reference:

http://developer.apple.com/documentation/MacOSX/Conceptual/universal_binary/universal_binary_diffs/chapter_3_section_9.html

[nate]

Aha!  I should have thought to check out the errata!  Thanks for the pointers to the errata and reference link.

[mdeh]

Aha!  I should have thought to check out the errata!  Thanks for the pointers to the errata and reference link.

You are most welcome