blocks|key|1261461|text|你也可以使用常量来做这件事。但是没有检查。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1261462|class+LibraryItem+<+Object
++class+<<+self;+attr_accessor+:attributes;+end
++ATTRIBUTES+=+['title',+'authors',+'location',]
end

class+LibraryBook+<+LibraryItem
++ATTRIBUTES+.push('ISBN',+'pages']
end|code-block|syntax|javascript|1261463|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

You can do it using CONSTANTS also. No check though. 

<pre><code>class LibraryItem &lt; Object
 class &lt;&lt; self; attr_accessor :attributes; end
 ATTRIBUTES = ['title', 'authors', 'location',]
end

class LibraryBook &lt; LibraryItem
 ATTRIBUTES .push('ISBN', 'pages']
end
</code></pre>

blocks|key|1261486|text|就像一个版本：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1261487|class+LibraryItem+<+Object
++def+initialize
++++@attributes+=+['one',+'two'];
++end
end

class+LibraryBook+<+LibraryItem
++def+initialize
+++super
+++@attributes.push('three')
+end
end

b+=+LibraryBook.new|code-block|syntax|javascript|1261488|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Just as a version:

<pre><code>class LibraryItem &lt; Object
 def initialize
 @attributes = ['one', 'two'];
 end
end

class LibraryBook &lt; LibraryItem
 def initialize
 super
 @attributes.push('three')
 end
end

b = LibraryBook.new
</code></pre>

blocks|key|1046660|text|在属性中，@attributes是一个新的自变量，对象LibraryBook的实例变量，所以它没有初始化，你会得到错误"undefined+method+...+for+nil“|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1046661|在使用之前，应该通过LibraryItem属性的列表对其进行初始化|1046662|class+LibraryBook+<+LibraryItem
++@attributes+=+LibraryItem::attributes+%2B+['ISBN',+'pages']
end|code-block|syntax|javascript|1046663|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

In LibraryBook variable @attributes is a new independent variable, instance variable of object LibraryBook, so its not initialized and you get error "undefined method ... for nil" 
You should to initialize it by LibraryItem attribut's list before using

<pre><code>class LibraryBook &lt; LibraryItem
 @attributes = LibraryItem::attributes + ['ISBN', 'pages']
end
</code></pre>

blocks|key|1261607|text|出于好奇，像这样的东西会起作用吗？|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1261608|class+Foo
++ATTRIBUTES+=+['title','authors','location']
end

class+Bar+<+Foo
++ATTRIBUTES+%7C=+['ISBN',+'pages']
end|code-block|syntax|javascript|1261609|这似乎会产生所需的结果-属性数组在创建类对象时展开，属性的值按预期变化：|1261610|>+Foo::ATTRIBUTES
=>+['title','authors','location']+
>+Bar::ATTRIBUTES
=>+['title','authors','location',+'ISBN',+'pages']+|1261611|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]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

Out of curiosity, will something like this work?

<pre><code>class Foo
 ATTRIBUTES = ['title','authors','location']
end

class Bar &lt; Foo
 ATTRIBUTES |= ['ISBN', 'pages']
end
</code></pre>

This would seem to produce the desired result - the ATTRIBUTES array is expanded when the class object is created, and the values of ATTRIBUTES varies as expected:

<pre><code>&gt; Foo::ATTRIBUTES
=&gt; ['title','authors','location'] 
&gt; Bar::ATTRIBUTES
=&gt; ['title','authors','location', 'ISBN', 'pages'] 
</code></pre>

blocks|key|1046805|text|既然你提到属性是“固定的”和“不变的”，我假设你的意思是，一旦创建了对象，你就永远不会改变它们的值。在这种情况下，应该可以使用如下代码：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1046806|class+Foo
++++ATTRS+=+['title',+'authors',+'location']
++++def+attributes
++++++++ATTRS
++++end
end

class+Bar+<+Foo
++++ATTRS+=+['ISBN',+'pages']
++++def+attributes
++++++++super+%2B+ATTRS
++++end
end|code-block|syntax|javascript|1046807|您正在手动实现一个读取器方法(而不是让attr_accessor为您创建它)，该方法伪装了数组的内部名称。在您的子类中，您只需调用祖先类的读取器函数，添加与子类相关的附加字段，并将其返回给调用者。对于用户来说，这就像一个名为attributes的只读成员变量，它在子类中具有其他值。|offset|length|style|CODE|1046808|entityMap^0|0|0|J|D|34|A|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@$I|R|J|S|K|L]|$I|T|J|U|K|L]]|9|@]|A|$]]|$1|M|3|-4|5|6|7|V|8|@]|9|@]|A|$]]]|N|$]]

Since you mention that the attributes are "fixed" and "unchanging", I am assuming that you mean that you will never change their value once the object is created. In that case, something like the following should work:

<pre><code>class Foo
 ATTRS = ['title', 'authors', 'location']
 def attributes
 ATTRS
 end
end

class Bar &lt; Foo
 ATTRS = ['ISBN', 'pages']
 def attributes
 super + ATTRS
 end
end
</code></pre>

You are manually implementing a reader method (instead of letting <code>attr_accessor</code> create it for you) that disguises the internal name of the array. In your subclass, you simply call the ancestor class' reader function, tack on the additional fields associated with the child class, and return that to the caller. To the user, this appears like a read-only member variable named <code>attributes</code> that has additional values in the sub-class.

blocks|key|1046897|text|ActiveSupport在rails边缘有class_attribute方法。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1046898|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|$]]

ActiveSupport has class_attribute method in rails edge.

blocks|key|1046947|text|为了扩展@Nick+Vanderbilt的答案，使用active_support可以做到这一点，这正是我想要的这个功能的速记。下面是一个完整的示例：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1046948|require+'active_support/core_ext'

class+Foo
++class_attribute+:attributes
++self.attributes+=+['title','authors','location']
end

class+Bar+<+Foo
++self.attributes+=+Foo.attributes+%2B+['ISBN',+'pages']
end

puts+Foo.attributes.inspect+#=>+["title",+"authors",+"location"]
puts+Bar.attributes.inspect+#=>+["title",+"authors",+"location",+"ISBN",+"pages"]|code-block|syntax|javascript|1046949|遗憾的是，ruby在不需要库的情况下很难做到这一点。这是我在python中唯一怀念的东西。在我的例子中，我并不介意对active_support+gem的依赖。|1046950|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

To expand on @Nick Vanderbilt's answer, using active_support you do this, which is exactly the short hand I want for this functionality. Here's a complete example:

<pre><code>require 'active_support/core_ext'

class Foo
 class_attribute :attributes
 self.attributes = ['title','authors','location']
end

class Bar &lt; Foo
 self.attributes = Foo.attributes + ['ISBN', 'pages']
end

puts Foo.attributes.inspect #=&gt; ["title", "authors", "location"]
puts Bar.attributes.inspect #=&gt; ["title", "authors", "location", "ISBN", "pages"]
</code></pre>

Shame it's so difficult for ruby to achieve this without needing a library for it. It's the only thing I miss from python. And in my case, I don't mind the dependency on the active_support gem.

blocks|key|1261877|text|这是字符串(实际上是任何东西)，而不是数组，但是...|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1261878|class+A
++def+self.a
++++@a+%7C%7C+superclass.a+rescue+nil
++end

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

++self.a+=+%25w(+apple+banana+chimp+)
end

class+B+<+A
end

class+C+<+B
++self.a+%2B=+%25w(+dromedary+elephant+)
end

class+D+<+A
++self.a+=+%25w(+pi+e+golden_ratio+)
end



irb(main):001:0>+require+'test2'
=>+true
irb(main):002:0>+A.a
=>+["apple",+"banana",+"chimp"]
irb(main):003:0>+B.a
=>+["apple",+"banana",+"chimp"]
irb(main):004:0>+C.a
=>+["apple",+"banana",+"chimp",+"dromedary",+"elephant"]
irb(main):005:0>+D.a
=>+["pi",+"e",+"golden_ratio"]
irb(main):006:0>+A.a+=+%25w(+7+)+
=>+["7"]
irb(main):007:0>+A.a
=>+["7"]
irb(main):008:0>+B.a
=>+["7"]
irb(main):009:0>+C.a+=+nil
=>+nil
irb(main):010:0>+C.a
=>+["7"]|code-block|syntax|javascript|1261879|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

This is for strings (anything really), rather than arrays, but...

<pre><code>class A
 def self.a
 @a || superclass.a rescue nil
 end

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

 self.a = %w( apple banana chimp )
end

class B &lt; A
end

class C &lt; B
 self.a += %w( dromedary elephant )
end

class D &lt; A
 self.a = %w( pi e golden_ratio )
end



irb(main):001:0&gt; require 'test2'
=&gt; true
irb(main):002:0&gt; A.a
=&gt; ["apple", "banana", "chimp"]
irb(main):003:0&gt; B.a
=&gt; ["apple", "banana", "chimp"]
irb(main):004:0&gt; C.a
=&gt; ["apple", "banana", "chimp", "dromedary", "elephant"]
irb(main):005:0&gt; D.a
=&gt; ["pi", "e", "golden_ratio"]
irb(main):006:0&gt; A.a = %w( 7 ) 
=&gt; ["7"]
irb(main):007:0&gt; A.a
=&gt; ["7"]
irb(main):008:0&gt; B.a
=&gt; ["7"]
irb(main):009:0&gt; C.a = nil
=&gt; nil
irb(main):010:0&gt; C.a
=&gt; ["7"]
</code></pre>

I have a Ruby class called <code>LibraryItem</code>. I want to associate with every instance of this class an array of attributes. This array is long and looks something like

<pre><code>['title', 'authors', 'location', ...]
</code></pre>

Note that these attributes are not really supposed to be methods, just a list of attributes that a <code>LibraryItem</code> has.

Next, I want to make a subclass of <code>LibraryItem</code> called <code>LibraryBook</code> that has an array of attributes that includes all the attributes of <code>LibraryItem</code> but will also include many more.

Eventually I will want several subclasses of <code>LibraryItem</code> each with their own version of the array <code>@attributes</code> but each adding on to <code>LibraryItem</code>'s <code>@attributes</code> (e.g., <code>LibraryBook</code>, <code>LibraryDVD</code>, <code>LibraryMap</code>, etc.).

So, here is my attempt: 

<pre><code>class LibraryItem &lt; Object
 class &lt;&lt; self; attr_accessor :attributes; end
 @attributes = ['title', 'authors', 'location',]
end

class LibraryBook &lt; LibraryItem
 @attributes.push('ISBN', 'pages')
end
</code></pre>

This does not work. I get the error 

<pre><code>undefined method `push' for nil:NilClass
</code></pre>

If it were to work, I would want something like this

<pre><code>puts LibraryItem.attributes 
puts LibraryBook.attributes
</code></pre>

to output 

<pre><code>['title', 'authors', 'location']
['title', 'authors', 'location', 'ISBN', 'pages']
</code></pre>

(Added 02-May-2010)
One solution to this is to make <code>@attributes</code> a simple instance variable and then add the new attributes for <code>LibraryBoot</code> in the <code>initialize</code> method (this was suggested by demas in one of the answers). 

While this would certainly work (and is, in fact, what I have been doing all along), I am not happy with this as it is sub-optimal: why should these unchanging arrays be constructed every time an object is created?

What I really want is to have class variables that can inherit from a parent class but when changed in the child class do not change in the the parent class.

Ruby class instance variables and inheritance

我有一个名为LibraryItem的Ruby类。我希望这个类的每个实例都有一个属性数组。这个数组很长，看起来像这样['title', 'authors', 'location', ...]请注意，这些属性并不是真正的方法，只是LibraryItem拥有的属性列表。接下来，我想创建一个名为LibraryBook的Libr...

问Ruby类实例变量和继承
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