The Obj-C function call [foo doSomething] compiles to the equivalent of objc_msgSend(foo, "doSomething"); (for the pedantic, I'm intentionally simplifying and leaving out objc_selfrefs)

The first thing objc_msgSend does is check if foo is nil and, if so, returns 0/nil/the all-zero bit pattern/whatever. It can be extremely convenient, but it can also be extremely inconvenient.

Calling any method ("sending any message" in Objective-C terminology) on a nil receiver does nothing and returns the zero value of the method's result type (a value of the right size with all bits zeroed).

In Objective-C 'nil' is a special object (internally rwpresented by a null pointer). When the runtime is instructed to invoke any method on nil, the method does nothing and returns nil (rather than crashing). I think this is an artifact of Smalltalk behavior.