『IOS』nil,NULL,NSNull的区别

主要就以下几点：
1、nil：一般赋值给空对象；
2、NULL：一般赋值给nil之外的其他空值。如SEL等；
　　举个栗子（好重啊~）：
　　　　[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
3、NSNULL：NSNull只有一个方法：+ (NSNull *) null;
　　[NSNull null]用来在NSArray和NSDictionary中加入非nil（表示列表结束）的空值.
4、当向nil发送消息时，返回NO，不会有异常，程序将继续执行下去；
　　而向NSNull的对象发送消息时会收到异常。
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以下转载自：http://blog.sina.com.cn/s/blog_722017670100v2b4.html

nil: A null pointer to an Objective-C object. ( #define nil ((id)0) )
nil 是一个对象值。

Nil: A null pointer to an Objective-C class.

NULL: A null pointer to anything else. ( #define NULL ((void *)0) )
NULL是一个通用指针（泛型指针）。

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.
[NSNull null]是一个对象，他用在不能使用nil的场合。
因为在NSArray和NSDictionary中nil中有特殊的含义（表示列表结束），所以不能在集合中放入nil值。如要确实需要存储一个表示“什么都没有”的值，可以使用NSNull类。NSNull只有一个方法：
+ (NSNull *) null;

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 *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]

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胖叔——zhulin1987.com