NULL vs nil in Objective-C

　　nil只能被應用在id類型的地方，就是Java與C++中指向對象的指標。而NULL用於非對象指標。

請看如下執行個體：

 -(void)observeValueForKeyPath NSString *)keyPath ofObject id)object

change:(NSDictionary *)change context:(void *)context
　　形參context是null 指標類型，是一個C類型指標，所以用NULL定義，null 指標類型有時被聲明為
(void *)0 ，而不是 id類型的nil.

原文：

"nil" should only be used in place of an "id", what we Java and C++
programmers would think of as a pointer to an object. Use NULL for
non-object pointers.

Look at the declaration of that method:

-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object
change:(NSDictionary *)change context:(void *)context

context is a "void *" (ie a C-style pointer), so you'd definitely use
NULL (which is sometimes declared as "(void *)0") rather than nil
(which is of type "id").