ruby 类变量上的 Attr_accessor

Attr_accessor on class variables

提问by Talespin_Kit

attr_accessordoes not work on the following code. The error says "undefined method 'things' for Parent:Class (NoMethodError)":

attr_accessor不适用于以下代码。错误说“ undefined method 'things' for Parent:Class (NoMethodError)”：

class Parent
  @@things = []
  attr_accessor :things
end
Parent.things << :car

p Parent.things

However the following code works

但是以下代码有效

class Parent
  @@things = []
  def self.things
    @@things
  end
  def things
    @@things
  end
end
Parent.things << :car

p Parent.things

回答by Alex.Bullard

attr_accessordefines accessor methods for an instance. If you want class level auto-generated accessors you could use it on the metaclass

attr_accessor为实例定义访问器方法。如果你想要类级自动生成的访问器，你可以在元类上使用它

class Parent
  @things = []

  class << self
    attr_accessor :things
  end
end

Parent.things #=> []
Parent.things << :car
Parent.things #=> [:car]

but note that this creates a class level instance variable nota class variable. This is likely what you want anyway, as class variables behave differently then you might expect when dealing w/ inheritence. See "Class and Instance Variables In Ruby".

但请注意，这会创建一个类级别的实例变量而不是类变量。无论如何，这可能是您想要的，因为类变量的行为与您在处理继承时可能期望的不同。请参阅“ Ruby 中的类和实例变量”。

回答by Chuck

attr_accessorgenerates accessors for instancevariables. Class variables in Ruby are a very different thing, and they are usually not what you want. What you probably want here is a class instance variable. You can use attr_accessorwith class instance variables like so:

attr_accessor为实例变量生成访问器。Ruby 中的类变量是非常不同的东西，它们通常不是您想要的。你可能想要的是一个类实例变量。您可以attr_accessor像这样使用类实例变量：

class Something
  class <<self
    attr_accessor :things
  end
end

Then you can write Something.things = 12and it will work.

然后你可以写Something.things = 12，它会起作用。

回答by Paul Byrne

This is probably the simplest way.

这可能是最简单的方法。

class Parent
  def self.things
    @@things ||= []
  end
end
Parent.things << :car

p Parent.things

回答by Vlad Khomich

Just some clarification: class variables won't be accessible using attr_accessor. It's all about instance variables:

只是一些澄清：类变量将无法使用attr_accessor. 这都是关于实例变量的：

class SomeClass
  class << self
    attr_accessor :things
  end
  @things = []
end

because in Ruby, class is an instance of the class "Class" (God, I love to say that) and attr_accessorsets accessor methods for instance variables.

因为在 Ruby 中，class 是类“Class”（上帝，我喜欢这么说）的一个实例，并attr_accessor为实例变量设置访问器方法。

回答by artamonovdev

Аlso note that a singleton methodis a method only for a single object. In Ruby, a Class is also an object, so it too can have singleton methods! So be aware of when you might be calling them.

还请注意，单例方法是仅用于单个对象的方法。在 Ruby 中，一个类也是一个对象，所以它也可以有单例方法！因此，请注意您可能会致电他们的时间。

Example:

例子：

class SomeClass
  class << self
    def test
    end
  end
end

test_obj = SomeClass.new

def test_obj.test_2
end

class << test_obj
  def test_3
  end
end

puts "Singleton methods of SomeClass"
puts SomeClass.singleton_methods
puts '------------------------------------------'
puts "Singleton methods of test_obj"
puts test_obj.singleton_methods

Singleton methods of SomeClass

SomeClass 的单例方法

test

测试

---

---

Singleton methods of test_obj

test_obj 的单例方法

test_2

测试_2

test_3

测试_3

回答by su_li

Parent.class_variable_get(:@@things)

That would be the built-in way. In most cases this should be sufficient I think. No need to have a class variable accessor in the instance.

那将是内置的方式。在大多数情况下，我认为这应该足够了。实例中不需要有类变量访问器。