Class variables and extending a module

Question

I have a module like the following

module MyModule
  def self.included(base)
    base.extend ClassMethods
  end

  module ClassMethods
    def foo
      @@var = 1
    end

    def bar
      puts @@var
    end
  end
end

class A
  include MyModule
  foo
end

class B < A; end

so that

 B.bar outputs '1'.

However, I would like to have .bar only be defined if .foo is called. I tried

module MyModule
  def self.included(base)
    base.extend ClassMethods
  end

  module ClassMethods
    def foo
      @@var = 1
      extend SingletonMethods
    end

  module SingletonMethods
    def bar
      puts @@var
    end
  end
end

The problem is that

B.bar

returns the error "uninitialized class variable @@var in MyModule::SingletonMethods". How can I make it so that a variable defined in .foo is available to .bar?

Sigurd · Accepted Answer · 2010-11-02 09:44:02Z

1

use mattr_accessor instead

answered Nov 2, 2010 at 9:44

Sigurd

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3 Comments

Andrew Grimm Over a year ago

Is mattr_accessor Rails only?

Sigurd Over a year ago

Yes, but the implementation is pretty straightforward - github.com/rails/rails/blob/master/activesupport/lib/…

rico_mac Over a year ago

this saved me a lot of time.

Aditya Advani · Accepted Answer · 2012-11-11 19:30:17Z

0

I was able to access class variables from a module using the self syntax

class User
  include Base
  @@attributes = [:slug, :email, :crypted_password]
end

module Base
  def self.included(base)
  base.extend ClassMethods
end

def headers
  if defined? self.attributes
    self.attributes
  end
 end
end

Now calling User.headers gives me the expected result of [:slug, :email, :crypted_password]

If anyone can shed more light on why this works exactly so in ruby, please let me know!

answered Nov 11, 2012 at 19:30

Aditya Advani

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