blocks|key|909113|text|@@表示一个类变量，即它可以被继承。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|909114|这意味着如果创建该类的子类，它将继承变量。因此，如果您有一个带有类变量@@number_of_wheels的Vehicle类，那么如果您创建了一个class+Car+<+Vehicle，那么它也将具有类变量@@number_of_wheels|909115|entityMap^0|0|2|0|Z|I|1I|7|21|J|2V|I|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]|$9|P|A|Q|B|C]|$9|R|A|S|B|C]|$9|T|A|U|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|V|8|@]|D|@]|E|$]]]|I|$]]

<code>@@</code> denotes a class variable, i.e. it can be inherited.

This means that if you create a subclass of that class, it will inherit the variable. So if you have a class <code>Vehicle</code> with the class variable <code>@@number_of_wheels</code> then if you create a <code>class Car &lt; Vehicle</code> then it too will have the class variable <code>@@number_of_wheels</code>

blocks|key|693127|text|@+-类的实例变量|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|693128|@@类变量，在某些情况下也称为静态变量|693129|类变量是在类的所有实例之间共享的变量。这意味着从这个类实例化的所有对象只有一个变量值。如果一个对象实例更改了变量的值，那么所有其他对象实例的新值都将发生本质上的变化。|693130|另一种思考类变量的方式是在单个类的上下文中作为全局变量。通过在变量名前加上两个@字符(@@)来声明类变量。类变量必须在创建时初始化|693131|entityMap^0|0|1|0|0|2|0|0|13|1|17|2|0^^$0|@$1|2|3|4|5|6|7|N|8|@$9|O|A|P|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Q|8|@$9|R|A|S|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|T|8|@]|D|@]|E|$]]|$1|J|3|K|5|6|7|U|8|@$9|V|A|W|B|C]|$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|L|3|-4|5|6|7|Z|8|@]|D|@]|E|$]]]|M|$]]

<code>@</code> - Instance variable of a class 
<code>@@</code> - Class variable, also called as static variable in some cases

A class variable is a variable that is shared amongst all instances of a class. This means that only one variable value exists for all objects instantiated from this class. If one object instance changes the value of the variable, that new value will essentially change for all other object instances.

Another way of thinking of thinking of class variables is as global variables within the context of a single class.
Class variables are declared by prefixing the variable name with two <code>@</code> characters (<code>@@</code>). Class variables must be initialized at creation time

blocks|key|693187|text|当类扩展或包含模块时，模块中的@和@@的工作方式也不同。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|693188|所以给出了|693189|module+A
++++@a+=+'module'
++++@@a+=+'module'

++++def+get1
++++++++@a++++++++++
++++end+++++

++++def+get2
++++++++@@a+++++++++
++++end+++++

++++def+set1(a)+
++++++++@a+=+a++++++
++++end+++++

++++def+set2(a)+
++++++++@@a+=+a+++++
++++end+++++

++++def+self.set1(a)
++++++++@a+=+a++++++
++++end+++++

++++def+self.set2(a)
++++++++@@a+=+a+++++
++++end+++++
end+|code-block|syntax|javascript|693190|然后，您将获得以下显示为注释的输出|693191|class+X
++++extend+A

++++puts+get1.inspect+#+nil
++++puts+get2.inspect+#+"module"

++++@a+=+'class'+
++++@@a+=+'class'+

++++puts+get1.inspect+#+"class"
++++puts+get2.inspect+#+"module"

++++set1('set')
++++set2('set')

++++puts+get1.inspect+#+"set"+
++++puts+get2.inspect+#+"set"+

++++A.set1('sset')
++++A.set2('sset')

++++puts+get1.inspect+#+"set"+
++++puts+get2.inspect+#+"sset"
end+

class+Y
++++include+A

++++def+doit
++++++++puts+get1.inspect+#+nil
++++++++puts+get2.inspect+#+"module"

++++++++@a+=+'class'
++++++++@@a+=+'class'

++++++++puts+get1.inspect+#+"class"
++++++++puts+get2.inspect+#+"class"

++++++++set1('set')
++++++++set2('set')

++++++++puts+get1.inspect+#+"set"
++++++++puts+get2.inspect+#+"set"

++++++++A.set1('sset')
++++++++A.set2('sset')

++++++++puts+get1.inspect+#+"set"
++++++++puts+get2.inspect+#+"sset"
++++end
end

Y.new.doit|693192|因此，在模块中对所有变量使用@@，在模块中对变量使用@，在每个使用上下文中分隔变量。|693193|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|$]]

@ and @@ in modules also work differently when a class extends or includes that module.

So given

<pre><code>module A
 @a = 'module'
 @@a = 'module'

 def get1
 @a 
 end 

 def get2
 @@a 
 end 

 def set1(a) 
 @a = a 
 end 

 def set2(a) 
 @@a = a 
 end 

 def self.set1(a)
 @a = a 
 end 

 def self.set2(a)
 @@a = a 
 end 
end 
</code></pre>

Then you get the outputs below shown as comments

<pre><code>class X
 extend A

 puts get1.inspect # nil
 puts get2.inspect # "module"

 @a = 'class' 
 @@a = 'class' 

 puts get1.inspect # "class"
 puts get2.inspect # "module"

 set1('set')
 set2('set')

 puts get1.inspect # "set" 
 puts get2.inspect # "set" 

 A.set1('sset')
 A.set2('sset')

 puts get1.inspect # "set" 
 puts get2.inspect # "sset"
end 

class Y
 include A

 def doit
 puts get1.inspect # nil
 puts get2.inspect # "module"

 @a = 'class'
 @@a = 'class'

 puts get1.inspect # "class"
 puts get2.inspect # "class"

 set1('set')
 set2('set')

 puts get1.inspect # "set"
 puts get2.inspect # "set"

 A.set1('sset')
 A.set2('sset')

 puts get1.inspect # "set"
 puts get2.inspect # "sset"
 end
end

Y.new.doit
</code></pre>

So use @@ in modules for variables you want common to all their uses, and use @ in modules for variables you want separate for every use context.

blocks|key|909319|text|答案是部分正确的，因为@@实际上是一个按类层次结构的类变量，这意味着它由一个类、它的实例、它的子类以及它们的实例共享。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|909320|class+Person
++@@people+=+[]

++def+initialize
++++@@people+<<+self
++end

++def+self.people
++++@@people
++end
end

class+Student+<+Person
end

class+Graduate+<+Student
end

Person.new
Student.new

puts+Graduate.people|code-block|syntax|javascript|909321|这将输出|909322|#<Person:0x007fa70fa24870>
#<Student:0x007fa70fa24848>|909323|因此，Person、Student和Graduate类只有一个相同的@@变量，这些类的所有类和实例方法都引用相同的变量。|909324|还有另一种定义class变量的方法，它是在class对象上定义的(请记住，每个类实际上都是某个东西的实例，实际上是class类，但这是另一回事)。您使用@+notation而不是@@，但是您不能从实例方法访问这些变量。你需要有类方法包装器。|909325|class+Person

++def+initialize
++++self.class.add_person+self
++end

++def+self.people
++++@people
++end

++def+self.add_person+instance
++++@people+%7C%7C=+[]
++++@people+<<+instance
++end
end

class+Student+<+Person
end

class+Graduate+<+Student
end

Person.new
Person.new
Student.new
Student.new
Graduate.new
Graduate.new

puts+Student.people.join(",")
puts+Person.people.join(",")
puts+Graduate.people.join(",")|909326|在这里，@people是每个类的一个，而不是类层次结构，因为它实际上是存储在每个类实例上的一个变量。这是输出：|909327|#<Student:0x007f8e9d2267e8>,#<Student:0x007f8e9d21ff38>
#<Person:0x007f8e9d226158>,#<Person:0x007f8e9d226608>
#<Graduate:0x007f8e9d21fec0>,#<Graduate:0x007f8e9d21fdf8>+|909328|一个重要的区别是，您不能直接从实例方法访问这些类变量(或者可以说是类实例变量)，因为实例方法中的@people将引用Person、Student或Graduate类的特定实例的实例变量。|909329|因此，虽然其他答案正确地指出@myvariable+(带有单个@符号)始终是一个实例变量，但这并不意味着它不一定是该类的所有实例的单个共享变量。|909330|entityMap^0|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|10|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|11|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|12|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|13|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|14|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|15|8|@]|9|@]|A|$]]|$1|O|3|P|5|D|7|16|8|@]|9|@]|A|$E|F]]|$1|Q|3|R|5|6|7|17|8|@]|9|@]|A|$]]|$1|S|3|T|5|D|7|18|8|@]|9|@]|A|$E|F]]|$1|U|3|V|5|6|7|19|8|@]|9|@]|A|$]]|$1|W|3|X|5|6|7|1A|8|@]|9|@]|A|$]]|$1|Y|3|-4|5|6|7|1B|8|@]|9|@]|A|$]]]|Z|$]]

The answers are partially correct because @@ is actually a class variable which is per class hierarchy meaning it is shared by a class, its instances and its descendant classes and their instances. 

<pre><code>class Person
 @@people = []

 def initialize
 @@people &lt;&lt; self
 end

 def self.people
 @@people
 end
end

class Student &lt; Person
end

class Graduate &lt; Student
end

Person.new
Student.new

puts Graduate.people
</code></pre>

This will output 

<pre><code>#&lt;Person:0x007fa70fa24870&gt;
#&lt;Student:0x007fa70fa24848&gt;
</code></pre>

So there is only one same @@variable for Person, Student and Graduate classes and all class and instance methods of these classes refer to the same variable. 

There is another way of defining a class variable which is defined on a class object (Remember that each class is actually an instance of something which is actually the Class class but it is another story). You use @ notation instead of @@ but you can't access these variables from instance methods. You need to have class method wrappers. 

<pre><code>class Person

 def initialize
 self.class.add_person self
 end

 def self.people
 @people
 end

 def self.add_person instance
 @people ||= []
 @people &lt;&lt; instance
 end
end

class Student &lt; Person
end

class Graduate &lt; Student
end

Person.new
Person.new
Student.new
Student.new
Graduate.new
Graduate.new

puts Student.people.join(",")
puts Person.people.join(",")
puts Graduate.people.join(",")
</code></pre>

Here, @people is single per class instead of class hierarchy because it is actually a variable stored on each class instance. This is the output:

<pre><code>#&lt;Student:0x007f8e9d2267e8&gt;,#&lt;Student:0x007f8e9d21ff38&gt;
#&lt;Person:0x007f8e9d226158&gt;,#&lt;Person:0x007f8e9d226608&gt;
#&lt;Graduate:0x007f8e9d21fec0&gt;,#&lt;Graduate:0x007f8e9d21fdf8&gt; 
</code></pre>

One important difference is that, you cannot access these class variables (or class instance variables you can say) directly from instance methods because @people in an instance method would refer to an instance variable of that specific instance of the Person or Student or Graduate classes. 

So while other answers correctly state that @myvariable (with single @ notation) is always an instance variable, it doesn't necessarily mean that it is not a single shared variable for all instances of that class.

What are Ruby variables preceded with double at signs (<code>@@</code>)? My understanding of a variable preceded with an at sign is that it is an instance variable, like this in PHP:

PHP version

<pre><code>class Person {

 public $name;

 public function setName($name) {
 $this-&gt;name = $name;
 }

 public function getName() {
 return $this-&gt;name;
 }
}
</code></pre>

Ruby equivalent

<pre><code>class Person

 def set_name(name)
 @name = name
 end

 def get_name()
 @name
 end
end
</code></pre>

What does the double at sign <code>@@</code> mean, and how does it differ from a single at sign?

What does @@variable mean in Ruby?

什么是前面有两个at符号的Ruby变量(@@)？我对带有at符号的变量的理解是它是一个实例变量，就像在PHP中这样：PHP版本class Person {    public $name;    public function setName($name) {        $this->name = $name;  ...

问@@variable在Ruby中是什么意思？
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