Objective-C中的nil与NULL
本文详细解释了Objective-C中nil、Nil与NULL的区别及应用场景。nil为Objective-C对象指针空值，Nil用于类指针空值，而NULL则适用于C风格内存指针。此外还介绍了NSNull类的用途及其singleton实例。
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.
Technically they're all the same,,, but in practice they give someone reading your code some hints about what's going on; just like naming classes with a capital letter and instances with lowercase is recommended, but not required.
If someone sees you passing NULL, they know the receiver expects a C pointer. If they see nil, they know the receiver is expecting an object. If they see Nil, they know the receiver is expecting a class. Readability.
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