Cleaning up with @try / @catch / @finally

This lesson covers cleanup with @try, @catch, and @finally in Objective-C, with the aim of running something whether or not an exception occurred.

A @finally block always runs, exception or no exception. Use it to close a file or to signal that work has finished.

In the sample code, divide() raises a division-by-zero error with @throw and catches it with @catch. The @finally block always shows "Ending the calculation", error or not.

A common early stumble is that Objective-C does not use exceptions very often. iOS convention handles most errors through NSError, so @finally is not an especially common construct either.

In professional work, exceptions are used in limited situations such as detecting fatal bugs, while ordinary errors go through NSError.

📖 Reference code

NSNumber *divide(int a, int b) {
    NSNumber *result = nil;
    @try {
        if (b == 0) {
            @throw [NSException exceptionWithName:@"DivideError" reason:@"cannot divide by zero" userInfo:nil];
        }
        result = @(a / b);
    } @catch (NSException *e) {
        NSLog(@"Error: %@", e.reason);
    } @finally {
        NSLog(@"Ending the calculation");
    }
    return result;
}

NSLog(@"%@", divide(10, 2));
divide(10, 0);

✍️ Your code

Type your code, then press "Run"

🧪 This site can't compile or run Objective-C directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).