关于nil,Nil,NULL区别

转自http://blog.csdn.net/yhawaii/article/details/7442529

nil: A null pointer to an Objective-C object.
( #define nil ((id)0)  )

Nil: A null pointer to an Objective-C class.

NULL: A null pointer to anything else,  is for C-style memory pointers.
( #define NULL ((void *)0)  )

NSNull: A class defines a singleton object used to represent null values in collection objects (which don't allow nil values).
[NSNull null]: The singleton instance of NSNull.

 

1.NSArray这样的类型，nil或NULL不能添加到其中，可以用 [NSNull null] 返回的对象来代替.(NSArray 已nil作为标识符)