Ruby Singleton attr_accessor

Question

I'm trying to create a class that includes the Singleton module while using attr_accessor. This doesn't seem to be working though.

require 'singleton'

class Foo
  attr_accessor :bar
  include Singleton
end

Foo.bar = 'foobar'

This gives the error:

undefined method `bar=' for Foo:Class (NoMethodError)

What am I doing wrong here?

you are calling a class method instead of an instance method which you don't have implemented. Singelton is a class with only one existent instance. You should be doing the call on an instance and not on the class. — egghese
– egghese, Commented Jul 9, 2013 at 13:25

sawa · Accepted Answer · 2013-07-09 13:28:17Z

5

Singleton does not change the way attr_accessor works. It only ensures that you can have only one instance. So you have to call attr_accessor within the eigenclass, just as you do without Singleton.

class Foo
  singleton_class.class_eval{attr_accessor :bar}
end

answered Jul 9, 2013 at 13:28

sawa

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3 Comments

Kyle Decot Over a year ago

Thanks. I really like this solution because people that use my gem won't have to do Foo.instance.blah...

Cary Swoveland Over a year ago

Note instance_eval or class_eval can be used here. Alternatively, one could write singleton_class.public_send(:attr_accessor, :bar).

phil pirozhkov Over a year ago

Yet another option is to open the singleton class: class Foo include Singleton class << self attr_accessor :bar end end

Arup Rakshit · Accepted Answer · 2013-07-09 13:36:12Z

2

Try -

     Foo.instance.bar = 'foobar'

Documented Here and Here

edited Jul 9, 2013 at 13:36

answered Jul 9, 2013 at 13:26

Arup Rakshit

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Comments

toro2k · Accepted Answer · 2013-07-09 13:36:24Z

1

Methods defined using attr_accessors are instance methods, you are trying to call the method bar= on the Foo class, that doesn't define it. You have to get the instance of Foo first and then call bar= on it:

Foo.instance.bar = "whatever"

answered Jul 9, 2013 at 13:36

toro2k

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