NULL vs nil in ObjectiveC

　　nil只能被应用在id类型的地方，就是Java与C++中指向对象的指针。而NULL用于非对象指针。

请看如下实例：

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

change:(NSDictionary *)change context:(void *)context
　　形参context是空指针类型，是一个C类型指针，所以用NULL定义，空指针类型有时被声明为
(void *)0 ，而不是 id类型的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").