日期：2011.05.27转：http://blog.sinzy.net/jinjian/entry/22022nil: 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/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 *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]
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