cvar.rb

#!/usr/bin/env ruby

module ClassInstanceVariables
  def class_instance_variable(name, value = nil, private: true)
    (class << self; self; end).instance_eval {
      attr_accessor name
      private name if private
    }
    public_send("#{name}=", value)

    attr_writer name
    define_method(name) { value }
    private name if private
  end
end

class Foo
  extend ClassInstanceVariables

  class_instance_variable :foo
  class_instance_variable :baz, 87, private: false
end

raise unless Foo.send(:foo) == nil
raise unless Foo.new.send(:foo) == nil
raise unless Foo.baz == 87
raise unless Foo.new.baz == 87