NSNull, Nil, nil, NULL区别（OC）

参考：http://nshipster.com/nil/

直接给个表吧

Symbol	Value	Meaning
NULL	(void *)0	literal null value for C pointers
nil	(id)0	literal null value for Objective-C objects
Nil	(Class)0	literal null value for Objective-C classes
NSNull	[NSNull null]	singleton object used to represent null