摘要：
nil: Anullpointer to an Objective-C object.
( #definenil((id)0) )
Nil: Anullpointer to an Objective-C class.
NULL: Anullpointer to anything else. ( #defineNULL((void *)0) )
NSNull: A class defines a singleton object used to representnullvalues in collection objects (which don't allownilvalues).
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