Message Forwarding

What happens if objc_msgSend gets to the top of the object hierarchy without finding a method implementation that corresponds to the selector in the message? If that happens, objc_msgSend calls the forwardInvocation: method of NSObject with an NSInvocation argument (see the sidebar NSInvocation) formed from the original message expression. NSObject’s implementation of forwardInvocation: invokes the NSObject method doesNotRecognizeSelector:.

The NSObject’s implementation of doesNotRecognizeSelector: raises an exception and logs an error message, and then your program crashes.