blocks|key|1217053|text|您可以在任何可以使用null的地方使用nil。主要区别在于，您可以将消息发送到nil，因此您可以在null无法工作的一些地方使用它。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1217054|一般来说，只要使用nil即可。|1217055|entityMap^0|A|4|J|3|13|3|1D|4|0|9|3|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]|$9|Q|A|R|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|V|8|@]|D|@]|E|$]]]|I|$]]

You can use <code>nil</code> about anywhere you can use <code>null</code>. The main difference is that you can send messages to <code>nil</code>, so you can use it in some places where <code>null</code> cant work.

In general, just use <code>nil</code>.

blocks|key|1431941|text|它们的类型不同。它们都是零，但是NULL是一个void+*，nil是一个id，而Nil是一个类指针。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1431942|entityMap^0|G|4|N|6|U|3|10|2|14|3|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]|$9|Q|A|R|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|S|8|@]|D|@]|E|$]]]|G|$]]

They differ in their types. They're all zero, but <code>NULL</code> is a <code>void *</code>, <code>nil</code> is an <code>id</code>, and <code>Nil</code> is a Class pointer.

blocks|key|1217152|text|NULL和nil彼此相等，但是nil是一个对象值，而NULL是一个通用指针值(具体来说，是(void*)0)。[NSNull+null]是一个对象，用于在不允许使用nil的情况下代替nil。例如，在NSArray中不能有nil值。因此，如果需要表示"nil"，可以使用[NSNull+null]。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1217153|entityMap^0|19|8|1J|D|3Q|D|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|O|8|@]|D|@]|E|$]]]|G|$]]

NULL and nil are equal to each other, but nil is an object value while NULL is a generic pointer value (<code>(void*)0</code>, to be specific). <code>[NSNull null]</code> is an object that's meant to stand in for nil in situations where nil isn't allowed. For example, you can't have a nil value in an NSArray. So if you need to represent a "nil", you can use <code>[NSNull null]</code>.

blocks|key|1217191|text|要扩展来自@cobbal的评论：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1217192|MacTypes.h包含：|offset|length|1217193|#ifndef+nil
+++#define+nil+NULL
#endif|code-block|syntax|javascript|1217194|entityMap|0|LINK|mutability|MUTABLE|url|http://opensource.apple.com/source/CarbonHeaders/CarbonHeaders-18.1/MacTypes.h^0|0|0|A|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|T|8|@]|9|@$D|U|E|V|1|W]]|A|$]]|$1|F|3|G|5|H|7|X|8|@]|9|@]|A|$I|J]]|$1|K|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]]|L|$M|$5|N|O|P|A|$Q|R]]]]

To expand on a comment from @cobbal: 

<a href="http://opensource.apple.com/source/CarbonHeaders/CarbonHeaders-18.1/MacTypes.h">MacTypes.h</a> contains:

<pre><code>#ifndef nil
 #define nil NULL
#endif
</code></pre>

blocks|key|1217209|text|如前所述，它们是相同的，但我使用其中一个或另一个取决于编写相应框架的语言。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1217210|对于与Objective-C相关的所有内容，我都使用nil。例如：|1217211|-+(BOOL)doSomethingWithObjectsInArray:(NSArray+*)anArray+{
++++if+(anArray+==+nil)+return+NO;

++++//+process+elements
++++...
}|code-block|syntax|javascript|1217212|但是，在检查来自C框架(如AddressBook框架和CoreFoundation)的数据模型的有效性时，我使用NULL。例如：|1217213|-+(BOOL)showABUnknownPersonVCForABRecordRef:(ABRecordRef)aRecord+{
++++if+(aRecord+==+NULL)+return+NO;

++++//+set-up+the+ABUnknownPersonViewController+and+display+it+on+screen
++++..
}|1217214|这样，如果我在处理Obj-C或基于C的代码，我的代码中就会有一些微妙的线索。|1217215|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|T|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|U|8|@]|9|@]|A|$G|H]]|$1|M|3|N|5|6|7|V|8|@]|9|@]|A|$]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

As already mentioned, they are the same, but I use either the one or the other depending on the language in which the corresponding framework was written.

For everything related to Objective-C, I use nil. For example:

<pre><code>- (BOOL)doSomethingWithObjectsInArray:(NSArray *)anArray {
 if (anArray == nil) return NO;

 // process elements
 ...
}
</code></pre>

However, when checking validity of data models from a C-framework (like AddressBook framework and CoreFoundation), I use NULL. For example:

<pre><code>- (BOOL)showABUnknownPersonVCForABRecordRef:(ABRecordRef)aRecord {
 if (aRecord == NULL) return NO;

 // set-up the ABUnknownPersonViewController and display it on screen
 ..
}
</code></pre>

This way, I have subtle clues in my code if I'm dealing with Obj-C or C based code.

blocks|key|1432137|text|我发现了以下内容：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1432138|objc.h|1432139|#define+Nil+__DARWIN_NULL+++/*+id+of+Nil+class+*/
#define+nil+__DARWIN_NULL+++/*+id+of+Nil+instance+*/|code-block|syntax|javascript|1432140|_types.h|1432141|#define+__DARWIN_NULL+((void+*)0)|1432142|stddef.h|1432143|#undef+NULL
#ifdef+__cplusplus
#undef+__null++//+VC%2B%2B+hack.
#define+NULL+__null
#else
#define+NULL+((void*)0)
#endif|1432144|MacTypes.h|1432145|#ifndef+NULL
#define+NULL++++__DARWIN_NULL
#endif+/*+!+NULL+*/
#ifndef+nil
++++#define+nil+NULL
#endif+/*+!+nil+*/|1432146|从外观上看，没有区别，只是概念上的区别。|1432147|entityMap^0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Z|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|10|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|11|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|12|8|@]|9|@]|A|$G|H]]|$1|M|3|N|5|6|7|13|8|@]|9|@]|A|$]]|$1|O|3|P|5|F|7|14|8|@]|9|@]|A|$G|H]]|$1|Q|3|R|5|6|7|15|8|@]|9|@]|A|$]]|$1|S|3|T|5|F|7|16|8|@]|9|@]|A|$G|H]]|$1|U|3|V|5|6|7|17|8|@]|9|@]|A|$]]|$1|W|3|-4|5|6|7|18|8|@]|9|@]|A|$]]]|X|$]]

I've found the following:

objc.h

<pre><code>#define Nil __DARWIN_NULL /* id of Nil class */
#define nil __DARWIN_NULL /* id of Nil instance */
</code></pre>

_types.h

<pre><code>#define __DARWIN_NULL ((void *)0)
</code></pre>

stddef.h

<pre><code>#undef NULL
#ifdef __cplusplus
#undef __null // VC++ hack.
#define NULL __null
#else
#define NULL ((void*)0)
#endif
</code></pre>

MacTypes.h

<pre><code>#ifndef NULL
#define NULL __DARWIN_NULL
#endif /* ! NULL */
#ifndef nil
 #define nil NULL
#endif /* ! nil */
</code></pre>

The way it looks, there's no difference but a conceptual one.

blocks|key|1217310|text|它们都是类型转换为0的，在功能上，它们之间没有区别。即，|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1217311|#define+NULL+((void*)0)
#define+nil+((id)0)|code-block|syntax|javascript|1217312|这是有区别的，但只对你自己和其他阅读代码的人来说，编译器并不关心。|1217313|另外，nil是一个对象值，而NULL是一个泛型指针值。|1217314|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|P|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

They both are just typecast zero's. Functionally, there's no difference between them.
ie.,

<pre><code>#define NULL ((void*)0)
#define nil ((id)0)
</code></pre>

There is a difference, but only to yourself and other humans that read the code, the compiler doesn't care.

One more thing nil is an object value while NULL is a generic pointer value.

blocks|key|1217359|text|例如，当您使用类型为(NSError+**)的输出参数调用Objective-C方法时，请使用NULL。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1217360|我在web上看到了很多示例代码，在这种情况下，人们提供的是nil而不是NULL。这是因为它是指向指针的指针，因此不是直接的Objective-C对象类型。如上所述，nil应该用于Objective-C对象类型。|1217361|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

Use NULL for example when you invoke an Objective-C method with an output parameter of type (NSError **).

I see lots of example code on the web where people provide nil instead of NULL in this case. This is because it's a pointer to a pointer and thus not directly an Objective-C object type. As said above, nil should be used for Objective-C object types.

blocks|key|1432238|text|在某些情况下是有区别的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1432239|从字面上看，Null是一个字符:+ASCII+0。|1432240|Nil等同于空，没有值。|1432241|根据编程环境的不同，这可能会有很大的不同。|1432242|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|L|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|M|8|@]|9|@]|A|$]]|$1|H|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|I|$]]

There is a difference in some contexts. 

Literally, Null is a character: ASCII 0. 

Nil is equivalent to blank, no value. 

Depending on the programming context, this can be a big difference.

blocks|key|1217428|text|注意，if([NSNull+null])返回true。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1217429|entityMap^0|3|H|M|4|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|G|$]]

Beware that <code>if([NSNull null])</code> returns <code>true</code>.

blocks|key|1432332|text|nil表示没有值，而NULL表示无对象，|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1432333|NSArray+*array+=+@[@"Hello+World+!"，@101，NSNULL+null+]；|1432334|这里NSNULL+null是一个对象，表示没有对象，同时您不能添加nil来表示缺少对象。|1432335|您也可以使用nil和NSNUll+null进行检查。|1432336|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|L|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|M|8|@]|9|@]|A|$]]|$1|H|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|I|$]]

nil means absence of value while NULL represent No Object,

NSArray *array = @[@"Hello World !", @101,[NSNULL null] ];

Here [NSNULL null] is an object which means no object, at the same time you cannot add nil to indicate absence of object.

you can use both nil and [NSNUll null] for checking too.

blocks|key|1217491|text|基本上:+nil:对象上的空指针，null:用于其他类型的指针|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1217492|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Basically:
nil: null pointer on an object and 
null: is for other type pointer

This is sample code:

<pre><code>NSDictionary *myDictionary = [NSDictionary dictionary];
NSNumber *myNumber = [myDictionary valueForKey: @"MyNumber"];
NSLog(@"myNumber = %@", myNumber); // output myNumber = (null)

if (myNumber == nil)
 NSLog(@"test 1 myNumber == nil");

if (myNumber == NULL)
 NSLog(@"test 2 myNumber == NULL");

if ([myNumber isEqual:[NSNull null]])
 NSLog(@"test 3 myNumber == [NSNull null]");
</code></pre>

When should I use nil, NULL and [NSNull null]?

When should I use nil and NULL in Objective-C?

Objective-C

编译器

以下是示例代码：NSDictionary *myDictionary = [NSDictionary dictionary];NSNumber *myNumber = [myDictionary valueForKey: @"MyNumber"];NSLog(@"myNumber = %@", myNumber); /...

问什么时候应该在Objective-C中使用nil和NULL？
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问什么时候应该在Objective-C中使用nil和NULL？
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