Why is it that sending any selector to a Nil object does nothing, but sending an “invalid” selector to any NSObject raises an exception?

Question

Does anyone know why NextStep/Apple decided to take the \"convenient method\" of doing nothing when passing a Nil object a message, but the \"Java method\" of raising an exc

Answer 1

From the good ol' documentation:

In Objective-C, it is valid to send a message to nil—it simply has no effect at runtime.

As for the other problem of the unrecognized selector behavior, an old implementation file of NSObject (from the MySTEP library) shows that the culprit is the NSObject method -doesNotRecognizeSelector:, which looks a bit as follows:

- (void) doesNotRecognizeSelector:(SEL)aSelector
{
    [NSException raise:NSInvalidArgumentException
                format:@"NSObject %@[%@ %@]: selector not recognized", 
                        object_is_instance(self)?@"-":@"+",
                        NSStringFromClass([self class]), 
                        NSStringFromSelector(aSelector)];
}

Which means that ObjC methods could feasibly be tinkered with so that they do not in fact have to raise an error. Which means the decision was entirely arbitrary, just like the decision to switch to "method-eating" messages to nil. A feat which can be done through method swizzling NSObject (wholly dangerous, as it will raise an EXC_BAD_ACCESS, or EXC_I386_BPT on mac, but at least it doesn't raise an exception)

void Swizzle(Class c, SEL orig, SEL new)
{
    Method origMethod = class_getInstanceMethod(c, orig);
    Method newMethod = class_getInstanceMethod(c, new);
    if(class_addMethod(c, orig, method_getImplementation(newMethod), method_getTypeEncoding(newMethod)))
        class_replaceMethod(c, new, method_getImplementation(origMethod), method_getTypeEncoding(origMethod));
    else
        method_exchangeImplementations(origMethod, newMethod);
}

-(void)example:(id)sender {
    Swizzle([NSObject class], @selector(doesNotRecognizeSelector:), @selector(description));
    [self performSelector:@selector(unrecog)];
}

The category:

@implementation NSObject (NoExceptionMessaging)

-(void)doesNotRecognizeSelector:(SEL)aSelector {
    NSLog(@"I've got them good ol' no exception blues.");
}
@end

Answer 2

For everyone's amusement, due to the discussion CodaFi and I were having, here's a quickly-hacked-together way to eat normally unresponded-to messages and have them return nil:

@interface EaterOfBadMessages : NSObject 
@end

@implementation EaterOfBadMessages

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector 
{
    NSMethodSignature * sig = [super methodSignatureForSelector:aSelector];
    if( !sig ){
        sig = [NSMethodSignature signatureWithObjCTypes:"@@:"];
    }
    return sig;
}

- (void)forwardInvocation:(NSInvocation *)anInvocation 
{
    id nilPtr = nil;
    [anInvocation setReturnValue:&nilPtr];
}

@end

int main(int argc, const char * argv[])
{

    @autoreleasepool {

        EaterOfBadMessages * e = [[EaterOfBadMessages alloc] init];
        // Of course, pre-ARC you could write [e chewOnThis]
        NSLog(@"-[EaterOfBadMessages chewOnThis]: %@", [e performSelector:@selector(chewOnThis)]);

    }
    return 0;
}

Please don't use this in real life.

Answer 3

I can't fully answer your question, but I can answer part of it. Objective-C allows you to send a message to nil because it makes code more elegant. You can read about this design decision here, and I will steal its example:

Let's say you want to get the last phone number that some person dialed on her office phone. If you can't send messages to nil, you have to write it like this:

Office *office = [somePerson office];
// Person might not have an office, so check it...
if (office) {
    Telephone *phone = [office telephone];
    // The office might not have a telephone, so check it...
    if (phone) {
        NSString *lastNumberDialed = [phone lastNumberDialed];
        // The phone might be brand new, so there might be no last-dialed-number...
        if (lastNumberDialed) {
            // Use the number, for example...
            [myTextField setText:lastNumberDialed];
        }
    }
}

Now suppose you can send messages to nil (and always get nil back):

NSString *lastNumberDialed = [[[somePerson office] telephone] lastNumberDialed];
if (lastNumberDialed) {
    [myTextField setText:lastNumberDialed];
}

As for why sending an unrecognized selector to an object raises an exception: I don't know for sure. I suspect that it's far more common for this to be a bug than to be harmless. In my code, I only want an unrecognized selector to be silently ignored when I need to send an optional protocol message (e.g. sending an optional message to a delegate). So I want the system to treat it as an error, and let me be explicit in the relatively rare case when I don't want it to be an error.

Note that you can tinker (to some extent) with the handling of unrecognized selectors in your own classes, in a few different ways. Take a look at the forwardingTargetForSelector:, forwardInvocation:, doesNotRecognizeSelector:, and resolveInstanceMethod: methods of NSObject.