The correct way to use NSError objects in your Objective-C Code

errors.m

// A function that returns an object or nil if there's an error.
- (NSObject*) doSomethingComplexAndReturnObject:(NSString*) input error:(NSError**) error {
    
    // do some work..
    BOOL itWorked = YES;
    
    if (itWorked) {
        return [[NSObject alloc] init]; // Do better memory management than this please.
    } else {
        *error = [NSError errorWithDomain:@"com.mycompany.myapp" code:14 userInfo:[NSDictionary dictionaryWithObject:@"My error message" forKey:NSLocalizedDescriptionKey]];
        return nil;
    }
}

// This function demonstrates using the function above
- (void) useTheDoSomethingComplexFunction {
    NSError* error = nil;
    NSObject* ob = [self doSomethingComplexAndReturnObject:@"foobar" error:&error];
    if (ob) {
        // do the success thing!
    } else if (error) {
        NSLog(@"Can't do the success thing as we have an error %@", [error localizedDescription] );
    }
}

This is incorrect and prone to crashing. What happens when I pass in NULL instead of an NSError **? You need to say:

if (error) 
    *error = .....;