blocks|key|575174|text|这是“没什么”。但是，如果您尝试在NULL上调用一个方法，那么您可以向其发送消息而不会被终止。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|575175|您可以查看this问题，了解有关NULL与nil的更多信息|575176|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/557582/null-vs-nil-in-objective-c^0|H|4|0|G|4|L|3|5|4|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@$9|Q|A|R|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@$9|X|A|Y|1|Z]]|E|$]]|$1|H|3|-4|5|6|7|10|8|@]|D|@]|E|$]]]|I|$J|$5|K|L|M|E|$N|O]]]]

It's "nothing". But a "nothing" you can send messages to without getting killed as you would if you were trying to call a method on <code>NULL</code>.

You can look at <a href="https://stackoverflow.com/questions/557582/null-vs-nil-in-objective-c">this</a> question to have more info on <code>NULL</code> vs. <code>nil</code>

blocks|key|2025533|text|它是一个null+pointer+-指向"nothing“的指针。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|2025534|entityMap|0|LINK|mutability|MUTABLE|url|http://en.wikipedia.org/wiki/Null_(computer_programming)^0|4|C|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

It's a <a href="http://en.wikipedia.org/wiki/Null_(computer_programming)" rel="nofollow noreferrer">null pointer</a> - a pointer to "nothing".

blocks|key|617600|text|也许我只是忽略了显而易见的问题，但这似乎是一个语义问题。你可以用很多不同的词来描述它，但是你的描述已经足够好了。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|617601|http://en.wikipedia.org/wiki/Null_(computer_programming)|offset|length|617602|entityMap|0|LINK|mutability|MUTABLE|url^0|0|0|1K|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|N|8|@]|9|@$D|O|E|P|1|Q]]|A|$]]|$1|F|3|-4|5|6|7|R|8|@]|9|@]|A|$]]]|G|$H|$5|I|J|K|A|$L|C]]]]

Maybe I'm just missing the obvious but this seems like a semantic question. As in, you can use lots of different words to describe it, but your description works well enough already.

<a href="http://en.wikipedia.org/wiki/Null_(computer_programming)" rel="nofollow noreferrer">http://en.wikipedia.org/wiki/Null_(computer_programming)</a>

blocks|key|575221|text|nil只能与指针一起使用，nil表示指向nothing+(它的值为零)的指针|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|575222|NSString+*myString+=+nil;+//+myString+points+to+nothing
int+x+=+nil;+//+invalid,+"x"+is+not+a+pointer,+but+it+will+compile|code-block|syntax|javascript|575223|entityMap^0|0|3|D|3|K|7|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]|$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|T|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|U|8|@]|D|@]|E|$]]]|L|$]]

nil should only be used with pointers, and nil represents a pointer which points to nothing (it's value is zero)

<pre><code>NSString *myString = nil; // myString points to nothing
int x = nil; // invalid, "x" is not a pointer, but it will compile
</code></pre>

blocks|key|575062|text|它不是一个空对象，而是缺少任何对象。其余的答案涵盖了其他语义，所以我就到此为止:)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|575063|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|$]]

It's not an empty object, it's the lack of any object at all. The rest of the answers cover the other semantics, so I'll leave it at that :)

blocks|key|1878082|text|正式定义时，它就像Joshual定义的那样--一个指向nothing的指针，或者一个指向任何对象的指针。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1878083|从实际实现的角度来看，尤其是在处理数据结构和算法时，nill通常会表示数据结构中的一个标记或表示“无”的对象。例如，在红黑树中，从技术上讲，所有的叶节点都是"nill"，但它们仍然具有叶节点相同或相似的属性和操作(颜色、指向父节点的指针等)--在这些情况下，它实际上是一个“无对象”……如果这有任何意义的话。|1878084|因此，在形式上，它是一个指向虚无的指针，在实践中，它通常被视为虚无的表示，但它永远不会……空。|1878085|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|H|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|I|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|J|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|G|$]]

Formally defined, it is as Joshual defined - a pointer to nothing or a pointer to no object at all.

From a practical, implementation perspective, particularly when dealing with data structures and algorithms, a nill often will represent a sentinel in a data structure or an object that represents "nothing" for example, in a red-black tree, technically, all of the leaf nodes are "nill", but they still have the same or similar properties &amp; operations of a leaf node (color, pointer to a parent, etc.) - in those cases, it is really a "nothing object" ... if that makes any sense.

So, formally, it is a pointer to nothing, in practice, it is often treated as a representation of nothing, but it is never ... null.

blocks|key|2025678|text|思考nil的一种有用的方式是，假设它是一个“什么也不做”的空对象。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2025679|所谓“什么都不做”，我的意思是，你发送的任何消息都不会有副作用。|2025680|它是“空的”，因为当您向它请求任何属性的值时，它总是返回nil。所以它没有保存任何值-->它是空的。|2025681|(实际上，当您请求返回不是obj-c对象的类型的属性的值时，情况要稍微复杂一些。您将得到一个指针大小的0。因此，对于任何不大于sizeof(void*)的标量值，都会得到0。但是，如果您要求在32位系统上使用struct或double，则会得到未定义的结果。我已经写过关于这个here的文章了。)|2025682|entityMap|0|LINK|mutability|MUTABLE|url|http://vgable.com/blog/2008/05/31/messages-to-nowhere/^0|2|3|0|0|S|3|0|1F|1|1R|D|2D|1|2W|6|33|6|3U|4|0|0^^$0|@$1|2|3|4|5|6|7|T|8|@$9|U|A|V|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|W|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|X|8|@$9|Y|A|Z|B|C]]|D|@]|E|$]]|$1|J|3|K|5|6|7|10|8|@$9|11|A|12|B|C]|$9|13|A|14|B|C]|$9|15|A|16|B|C]|$9|17|A|18|B|C]|$9|19|A|1A|B|C]]|D|@$9|1B|A|1C|1|1D]]|E|$]]|$1|L|3|-4|5|6|7|1E|8|@]|D|@]|E|$]]]|M|$N|$5|O|P|Q|E|$R|S]]]]

One useful way of thinking about <code>nil</code> is imagining that it's the empty object that "does nothing".

By "does nothing" I mean, any message you send it won't have side effects.

It's "empty" in the sense that when you ask it for the value of any property, it always returns <code>nil</code>. So it's not holding any values --> it's empty. 
(Actually, it's a little more complicated than that, when you ask for the value of a property that returns a type that's NOT an obj-c object. You will get back a pointer-sized <code>0</code>. So for any scalar values that are no larger than <code>sizeof(void*)</code> you get <code>0</code>. But if you ask for a <code>struct</code> or a <code>double</code> on a 32 bit system, you get an undefined result. I've written about this <a href="http://vgable.com/blog/2008/05/31/messages-to-nowhere/" rel="nofollow noreferrer">here</a>.)

blocks|key|1875669|text|只是澄清一下，在Objective-C中，nil和null不是一回事。它们也不像在数学或其他编程语言中那样在Objective-C中表示相同的东西。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1875670|Nil实际上是0x0的一个特殊内存地址(至少编译器将其视为一个地址)。Nil用作已命名但未分配的对象的地址。这允许测试命名对象的存在/分配，并提供一种向可能不存在的对象发送消息的安全方法。虽然在数学和一些语言中，你可以将标量变量与nil进行比较，但在Objective-c中，你不应该这样做。您应该只比较对象的地址。|1875671|相比之下，NULL可以表示定义为NULL==0整数值的标准C，也可以表示在内存中具有特定地址的类NSNull的实际分配对象。然而，NSNull是一个单例对象，即每个应用程序只有一个。它用作其他对象的占位符，通常在集合中。您不能将其与任何其他对象进行比较。您只能检查特定指针是否指向单例NSNull对象。然而请注意，作为已分配的对象[NSNull+null]!=NULL，|offset|length|style|CODE|1875672|nil和NULL之间的混淆出现是因为分配NULL地址会将其分配给0x0的nil地址。这样的代码可以工作，但有时会导致一些微妙的问题。最好养成不要混淆两者的习惯。|1875673|因此，在Objective-C中，nil、NUll和NSNull是三个不同但重叠的概念，很容易混淆。Nil应用于地址，NULL应用于标量值，NSNull应用作已分配对象的占位符。|1875674|entityMap|0|LINK|mutability|MUTABLE|url|http://developer.apple.com/iphone/library/documentation/Cocoa/Conceptual/NumbersandValues/Articles/Null.html#//apple_ref/doc/uid/TP40005153^0|0|0|G|7|4L|J|1C|6|0|0|0|0^^$0|@$1|2|3|4|5|6|7|V|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|W|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|X|8|@$F|Y|G|Z|H|I]|$F|10|G|11|H|I]]|9|@$F|12|G|13|1|14]]|A|$]]|$1|J|3|K|5|6|7|15|8|@]|9|@]|A|$]]|$1|L|3|M|5|6|7|16|8|@]|9|@]|A|$]]|$1|N|3|-4|5|6|7|17|8|@]|9|@]|A|$]]]|O|$P|$5|Q|R|S|A|$T|U]]]]

Just to clarify, in Objective-C nil and null are not the same thing. Neither do they represent the same thing in Objective-C as they do in mathematics or other programming languages. 

Nil is actually a special memory address of 0x0 (at least the compiler treats it as an address.) Nil is used as the address of an object that is named but not allocated. This allows for test for the existence/allocation of named objects as well as providing a safe way to send messages to objects that might not exist. While in math and some languages you can compare scalar variables to nil, in Objective-c, you should not. You should only compare the address of objects. 

NULL by contrast can mean either the standard C defined as an integer value of<code>NULL==0</code> or it can represents an actual allocated object of the class <a href="http://developer.apple.com/iphone/library/documentation/Cocoa/Conceptual/NumbersandValues/Articles/Null.html#//apple_ref/doc/uid/TP40005153" rel="nofollow noreferrer">NSNull</a> with a specific address in memory. NSNull is however, a singleton object i.e. only one exist for every application. It is used as placeholder for other objects usually in collections. You cannot use it in comparisons with any other object. You can only check if a particular pointer points to the singleton NSNull object. Note however that as an allocated object<code>[NSNull null]!=NULL</code>,

The confusion between nil and NULL arises because assigning an address of NULL assigns it to the nil address of 0x0. Such code works but can cause subtle problem at times. It's best to get in the habit of not confusing the two.

So, in Objective-C, nil, NUll and NSNull are three different but overlapping concepts that are easy to confuse. Nil should be used for addresses, NULL should be used for scalar values and NSNull should be used as a placeholder for allocated objects.

blocks|key|617821|text|Objective-C对象|type|unstyled|depth|inlineStyleRanges|entityRanges|data|617822|首先，当你调用这个的时候：|617823|id+someObject+=+[NSArray+array];|code-block|syntax|javascript|617824|someObject不是直接的数组对象，只是指向数组对象的指针。也就是说，如果someObject等于0x1234，那么在内存的那个地址上就有一个对象。|offset|length|style|CODE|617825|这就是为什么|617826|id+someOtherObject+=+someObject;|617827|不复制对象。现在，两个指针都指向同一个对象。|617828|指向0x0的指针|617829|那么，nil是如何定义的呢？让我们来看一下源代码：|617830|objc.h|617831|#define+nil+__DARWIN_NULL++++/*+id+of+Nil+instance+*/|617832|_types.h|617833|#ifdef+__cplusplus
…
#else+/*+!+__cplusplus+*/
#define+__DARWIN_NULL+((void+*)0)
#endif+/*+__cplusplus+*/|617834|看起来nil是指向地址0x0的指针。|617835|那又怎么样？|617836|让我们看看Objective-C+Programming+Reference有什么要说的：|617837|617838|将消息发送到nil|blockquote|617839|617840|在Objective-C中，向nil发送消息是有效的-它在运行时根本不起作用。Cocoa中有几种模式利用了这一事实。从消息返回到nil的值也可能是有效的：…|617841|617842|返回值为nil、0或所有变量均初始化为0的struct。它是哪一个取决于预期的返回类型。对于发送到nil的消息，objective-c运行时中有一个显式检查，这意味着它真的很快。|617843|Nil、nil、NULL|617844|这是三种类型。下面是所有的定义：|617845|#define+Nil+__DARWIN_NULL+++/*+id+of+Nil+class+*/
#define+nil+__DARWIN_NULL+++/*+id+of+Nil+instance+*/
#define+NULL+__DARWIN_NULL
#define+__DARWIN_NULL+((void+*)0)|617846|可以看出，它们都是完全相同的。Nil和nil是由Objective-C定义的，NULL来自于C。|617847|那有什么区别呢？这只是风格的问题。它使代码更具可读性。|617848|617849|Nil用作不存在的类：Class+someClass+=+Nil.|unordered-list-item|617850|nil用作不存在的实例：id+someInstance+=+nil.|617851|NULL是指向不存在的内存部分的指针：char+*theString+=+NULL.|617852|617853|短的|617854|nil不是空对象，而是一个不存在的对象。如果空对象不存在，则方法-getSomeObject不会返回空对象，而是返回nil，这会告诉用户没有对象。|617855|也许这是有意义的：(两者都可以编译和运行。)|617856|if+(anObject+==+nil)+{+++//+One+cannot+compare+nothing+to+nothing,
+++++++++++++++++++++++++//+that+wouldn't+make+sense.

if+(anObject)+{++++++++++//+Correct,+one+checks+for+the+existence+of+anObject|617857|entityMap|0|LINK|mutability|MUTABLE|url|http://developer.apple.com/mac/library/documentation/cocoa/conceptual/ObjectiveC/Articles/ocObjectsClasses.html#//apple_ref/doc/uid/TP30001163-CH11-SW7^0|0|0|0|0|A|13|A|1F|6|0|0|0|0|0|3|3|0|0|0|0|0|3|3|0|0|5|X|0|0|0|0|0|0|0|4|3|L|6|1D|3|0|0|3|4|3|8|4|0|0|0|F|3|J|3|13|4|0|0|0|0|3|B|L|0|0|3|C|L|0|0|4|J|M|0|0|0|0|3|W|E|1M|3|0|0|0^^$0|@$1|2|3|4|5|6|7|2L|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|2M|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|2N|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|2O|8|@$K|2P|L|2Q|M|N]|$K|2R|L|2S|M|N]|$K|2T|L|2U|M|N]]|9|@]|A|$]]|$1|O|3|P|5|6|7|2V|8|@]|9|@]|A|$]]|$1|Q|3|R|5|F|7|2W|8|@]|9|@]|A|$G|H]]|$1|S|3|T|5|6|7|2X|8|@]|9|@]|A|$]]|$1|U|3|V|5|6|7|2Y|8|@]|9|@]|A|$]]|$1|W|3|X|5|6|7|2Z|8|@$K|30|L|31|M|N]]|9|@]|A|$]]|$1|Y|3|Z|5|6|7|32|8|@]|9|@]|A|$]]|$1|10|3|11|5|F|7|33|8|@]|9|@]|A|$G|H]]|$1|12|3|13|5|6|7|34|8|@]|9|@]|A|$]]|$1|14|3|15|5|F|7|35|8|@]|9|@]|A|$G|H]]|$1|16|3|17|5|6|7|36|8|@$K|37|L|38|M|N]]|9|@]|A|$]]|$1|18|3|19|5|6|7|39|8|@]|9|@]|A|$]]|$1|1A|3|1B|5|6|7|3A|8|@]|9|@$K|3B|L|3C|1|3D]]|A|$]]|$1|1C|3|-4|5|6|7|3E|8|@]|9|@]|A|$]]|$1|1D|3|1E|5|1F|7|3F|8|@]|9|@]|A|$]]|$1|1G|3|-4|5|6|7|3G|8|@]|9|@]|A|$]]|$1|1H|3|1I|5|6|7|3H|8|@]|9|@]|A|$]]|$1|1J|3|-4|5|6|7|3I|8|@]|9|@]|A|$]]|$1|1K|3|1L|5|6|7|3J|8|@$K|3K|L|3L|M|N]|$K|3M|L|3N|M|N]|$K|3O|L|3P|M|N]]|9|@]|A|$]]|$1|1M|3|1N|5|6|7|3Q|8|@$K|3R|L|3S|M|N]|$K|3T|L|3U|M|N]|$K|3V|L|3W|M|N]]|9|@]|A|$]]|$1|1O|3|1P|5|6|7|3X|8|@]|9|@]|A|$]]|$1|1Q|3|1R|5|F|7|3Y|8|@]|9|@]|A|$G|H]]|$1|1S|3|1T|5|6|7|3Z|8|@$K|40|L|41|M|N]|$K|42|L|43|M|N]|$K|44|L|45|M|N]]|9|@]|A|$]]|$1|1U|3|1V|5|6|7|46|8|@]|9|@]|A|$]]|$1|1W|3|-4|5|6|7|47|8|@]|9|@]|A|$]]|$1|1X|3|1Y|5|1Z|7|48|8|@$K|49|L|4A|M|N]|$K|4B|L|4C|M|N]]|9|@]|A|$]]|$1|20|3|21|5|1Z|7|4D|8|@$K|4E|L|4F|M|N]|$K|4G|L|4H|M|N]]|9|@]|A|$]]|$1|22|3|23|5|1Z|7|4I|8|@$K|4J|L|4K|M|N]|$K|4L|L|4M|M|N]]|9|@]|A|$]]|$1|24|3|-4|5|6|7|4N|8|@]|9|@]|A|$]]|$1|25|3|26|5|6|7|4O|8|@]|9|@]|A|$]]|$1|27|3|28|5|6|7|4P|8|@$K|4Q|L|4R|M|N]|$K|4S|L|4T|M|N]|$K|4U|L|4V|M|N]]|9|@]|A|$]]|$1|29|3|2A|5|6|7|4W|8|@]|9|@]|A|$]]|$1|2B|3|2C|5|F|7|4X|8|@]|9|@]|A|$G|H]]|$1|2D|3|-4|5|6|7|4Y|8|@]|9|@]|A|$]]]|2E|$2F|$5|2G|2H|2I|A|$2J|2K]]]]

<h2>Objective-C objects</h2>

First of all, when you call this:

<pre><code>id someObject = [NSArray array];
</code></pre>

<code>someObject</code> isn't the array object directly but only a pointer to it. That means, if <code>someObject</code> is equal to <code>0x1234</code> there's an object at that address in the memory.

That's the reason why

<pre><code>id someOtherObject = someObject;
</code></pre>

doesn't copy the object. Both pointers point now to the same object.

<h2>Pointer to 0x0</h2>

So, how is <code>nil</code> defined? Let's take a look at the source code:

objc.h

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

_types.h

<pre><code>#ifdef __cplusplus
…
#else /* ! __cplusplus */
#define __DARWIN_NULL ((void *)0)
#endif /* __cplusplus */
</code></pre>

Looks like <code>nil</code> is a pointer to the address 0x0.

<h2>So what?</h2>

Let's see what the <a href="http://developer.apple.com/mac/library/documentation/cocoa/conceptual/ObjectiveC/Articles/ocObjectsClasses.html#//apple_ref/doc/uid/TP30001163-CH11-SW7" rel="noreferrer">Objective-C Programming Reference</a> has to say:

<blockquote>
 <h3>Sending Messages to nil</h3>
 
 In Objective-C, it is valid to send a
 message to nil—it simply has no effect
 at runtime. There are several patterns
 in Cocoa that take advantage of this
 fact. The value returned from a
 message to nil may also be valid: …
</blockquote>

The returned values are either <code>nil</code>, 0 or a <code>struct</code> with all variables initialized to 0. Which one it is depends on the expected return type. There is an explicit check in the objective-c runtime for messages to <code>nil</code>, that means it's really fast.

<h2><code>Nil</code>, <code>nil</code>, <code>NULL</code></h2>

Those are the 3 types. Here are all the definitions:

<pre><code>#define Nil __DARWIN_NULL /* id of Nil class */
#define nil __DARWIN_NULL /* id of Nil instance */
#define NULL __DARWIN_NULL
#define __DARWIN_NULL ((void *)0)
</code></pre>

As can be seen, they are all exactly the same. <code>Nil</code> and <code>nil</code> are defined by Objective-C, <code>NULL</code> comes from C.

What's the difference then? It's only about style. It makes the code more readable.

<ul>
<li><code>Nil</code> is used as a non-existent class: <code>Class someClass = Nil</code>.</li>
<li><code>nil</code> is used as a non-existent instance: <code>id someInstance = nil</code>.</li>
<li><code>NULL</code> is a pointer to a non-existent memory part: <code>char *theString = NULL</code>.</li>
</ul>

<h2>Short</h2>

<code>nil</code> isn't an empty object but a non-existent one. A method <code>-getSomeObject</code> doesn't return an empty object if it doesn't exist but returns <code>nil</code> which tells the user that there is no object.

Maybe this makes sense: (Both would compile and run.)

<pre><code>if (anObject == nil) { // One cannot compare nothing to nothing,
 // that wouldn't make sense.

if (anObject) { // Correct, one checks for the existence of anObject
</code></pre>

Currently I understand it as a kind of "empty object". But what's it really?

What describes nil best? What's that really?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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目前我把它理解为一种“空对象”。但它到底是什么呢？

问什么是对nil最好的描述？那到底是什么？
EN

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问什么是对nil最好的描述？那到底是什么？EN