iOS开发知识体系——nil，Nil，NULL区别

reference: http://blog.sina.com.cn/s/blog_722017670100v2b4.html

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.  ( #define NULL ((void *)0)  ) eg.SEL、IMP ...

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.

example 1:
[NSApp beginSheet:sheet
              modalForWindow:mainWindow
              modalDelegate:nil //pointing to an object
              didEndSelector:NULL //pointing to a non object/class
              contextInfo:NULL]; //pointing to a non object/class

example 2: The values1 will only have obj1,
NSObject *obj1 = [NSObject new];
NSObject *obj2 = nil;
NSObject *obj3 = [NSObject new];
NSArray *values1 = [NSArray arrayWithObjects:obj1,obj2,obj3,nil];

example 3: The values2 have 3 objects and the second object is instance of NUSNull
NSObject *obj1 = [NSObject new];
NSObject *obj2 = [NSNull null];
NSObject *obj3 = [NSObject new];
NSArray *values2 = [NSArray arrayWithObjects:obj1,obj2,obj3,nil];

about testing:
if obj is nil , [obj message] will return NO, without NSException
if obj is NSNull , [obj message] will throw a NSException
testing NSNull: (NSNull *)obj == [NSNull null]