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) )
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.
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/classexample 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 *values1 = [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]