CODEWITHSUNDEEP
ruby
Talespin_Kit
Talespin_Kit

Reputation: 21877

Attr_accessor on class variables

attr_accessor does not work on the following code. The error says "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

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

p Parent.things

Upvotes: 79

Views: 65648

Answers (8)

Salvatore Mendola
Salvatore Mendola

Reputation: 1

I think what you want is class_attribute

class Base
  class_attribute :foo
  self.foo = '--UNSET--'

  def show
    puts self.foo
  end
end

class X < Base
  self.foo = 'XXX'
end

class Y < Base
  self.foo = 'YYY'
end

class Z < Base
end

puts X.foo # -> 'XXX'
puts Y.foo # -> 'YYY'
puts Z.foo # -> '--UNSET--'
X.new.show # -> 'XXX'
X.foo = 'x-x-x'
X.new.show # -> 'x-x-x'
Y.new.show # -> 'YYY'

Upvotes: 0

Chuck
Chuck

Reputation: 237010

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.

Upvotes: 19

Alex.Bullard
Alex.Bullard

Reputation: 5563

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".

Upvotes: 119

Sajad Rastegar
Sajad Rastegar

Reputation: 3154

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]

Upvotes: 1

su_li
su_li

Reputation: 317

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.

Upvotes: 1

artamonovdev
artamonovdev

Reputation: 2380

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

Upvotes: 1

Paul Byrne
Paul Byrne

Reputation: 1615

This is probably the simplest way.

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

p Parent.things

Upvotes: 5

Vlad Khomich
Vlad Khomich

Reputation: 5880

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.

Upvotes: 5

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