类变量上的 Attr_accessor

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最佳答案

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

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 not a class variable. This is likely what you want anyway, as class variables behave differently than you might expect when dealing w/ inheritance. See "Class and Instance Variables In Ruby".

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attr_accessor generates accessors for instance variables. 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_accessor with class instance variables like so:

class Something
class << self
attr_accessor :things
end
end

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

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

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_accessor sets accessor methods for instance variables.

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This is probably the simplest way.

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


p Parent.things

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Аlso note that a singleton method is 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.

Example:

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

test

Singleton methods of test_obj

test_2

test_3

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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.

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class Parent
@things = []
  

singleton_class.send(:attr_accessor, :things)
end

This pattern is most useful when you are defining accessors dynamically or creating them inside a method:

class Foo
def self.add_accessor(name)
singleton_class.send(:attr_accessor, name)
end
end


Foo.add_accessor :things
Foo.things = [:car]
Foo.things # => [:car]