blocks|key|1340384|text|Objective-C运行时可能需要将对象反弹到几个不同的函数，因此它需要对象引用，而不是对象本身。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1340385|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|$]]

The Objective-C runtime may need to bounce the object around to a couple different functions, so it wants the object reference, and not the object itself.

blocks|key|1340420|text|因为objc_msgSend()是这样声明的：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1340421|id+objc_msgSend(id+theReceiver,+SEL+theSelector,+...)|code-block|syntax|javascript|1340422|entityMap|0|LINK|mutability|MUTABLE|url|http://developer.apple.com/mac/library/DOCUMENTATION/Cocoa/Conceptual/ObjCRuntimeGuide/Articles/ocrtHowMessagingWorks.html^0|2|E|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@$A|R|B|S|1|T]]|C|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]]]

Because <a href="http://developer.apple.com/mac/library/DOCUMENTATION/Cocoa/Conceptual/ObjCRuntimeGuide/Articles/ocrtHowMessagingWorks.html" rel="noreferrer">objc_msgSend()</a> is declared like this:

<pre><code>id objc_msgSend(id theReceiver, SEL theSelector, ...)
</code></pre>

blocks|key|1340476|text|部分原因是你会得到空指针异常的左和右。向nil发送消息是允许的，而且通常是完全合法的(它什么也不做，也不会生成错误)。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1340477|但您可以将其视为类似于C%2B%2B的->表示法:它执行方法并在一块语法糖中取消对指针的引用。|1340478|entityMap^0|K|3|0|F|2|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|P|8|@]|D|@]|E|$]]]|I|$]]

Part of the reason is that you would get null pointer exceptions left and right. Sending a message to <code>nil</code> is allowed and often perfectly legitimate (it does nothing and does not generate an error). 

But you can think of it as analogous to C++'s <code>-&gt;</code> notation: It executes the method and dereferences the pointer in one piece of syntactic sugar.

blocks|key|1125423|text|你永远不要取消引用对象指针，句号。它们被定义为指针类型，而不仅仅是“对象类型”，这一事实是C语言传统的产物。它完全等同于Java的类型系统，在这种系统中，对象总是通过引用来访问的。在Java中，你永远不会去引用一个对象--事实上，你不能。你不应该把它们当作指针，因为在语义上，它们不是。它们只是对象引用。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1125424|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|$]]

You never dereference object pointers, period. The fact that they're typed as pointers rather than just "object types" is an artifact of the language's C heritage. It's exactly equivalent to Java's type system, where objects are always accessed through references. You never dereference an object in Java — in fact, you can't. You should not think of them as pointers, because semantically, they aren't. They're just object references.

blocks|key|1125471|text|Objective-C中的对象本质上是一个struct。struct的第一个成员是Class+isa+(+struct的总大小可以使用isa确定)。struct的后续成员可以包括实例变量。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1125472|当您声明一个Objective-C对象时，您总是将其声明为指针类型，因为运行库将您的对象提供给其他方法和函数；如果这些方法和函数更改了struct的任何成员(通过修改实例变量等)。它们将“应用”到对象的所有引用，而不仅仅是方法或函数中的局部引用。|1125473|entityMap^0|L|6|S|6|15|9|1H|6|1V|3|22|6|0|1V|6|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]|$9|S|A|T|B|C]|$9|U|A|V|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|W|8|@$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|Z|8|@]|D|@]|E|$]]]|I|$]]

An object in Objective-C is essentially a <code>struct</code>. The <code>struct</code>'s first member is the <code>Class isa</code> (the total size of the <code>struct</code> can be determined using <code>isa</code>). Subsequent members of the <code>struct</code> may include instance variables.

When you declare an Objective-C object, you always declare it as a pointer type because the runtime gives your object to other methods and functions; if these change any members of the <code>struct</code> (by modifying instance variables, etc.) they will &ldquo;apply&rdquo; to all references to your object, and not just locally within the method or function.

In Objective-C, why <code>[object doSomething]</code>? Wouldn't it be <code>[*object doSomething]</code> since you're calling a method on the object?, which means you should dereference the pointer?

Why [object doSomething] and not [*object doSomething]?

Objective-C

Java

在Objective-C中，为什么是[object doSomething]？既然你在对象上调用一个方法，它不是[*object doSomething]吗?这意味着你应该取消对指针的引用。

问为什么是[object doSomething]而不是[*object doSomething]？
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