setting class variables.rb

class Object
  def metaclass
    class << self; return self; end
  end
end

class Foo

  # 1. def within class definition   
  def self.set_a
    @@a = true
  end
end

# 2. def outside class definition
def Foo.set_b
  @@b = true
end

# 3. class_eval
Foo.class_eval do
  def self.set_c
    @@c = true
  end
end

# 4. instance_eval
Foo.instance_eval do
  def self.set_d
    @@d = true
  end
end

# 5. define_method on metaclass
Foo.metaclass.send :define_method, :set_e do
  @@e = true
end

# 6. Include module into metaclass
module SetF
  def set_f
    @@f = true
  end
end
Foo.metaclass.send :include, SetF

# 7. Extend with module
module SetG
  def set_g
    @@g = true
  end
end
Foo.extend SetG

# Output: 

print "Variable\tOn class?\tOn metaclass?\n\n"

("a".."g").each do |var|
  Foo.send("set_#{var}")

  print "@@" + var + "\t\t"
  print (Foo.class_variables.include?("@@#{var}") ? Foo.send(:class_variable_get, "@@#{var}").to_s : "-") + "\t\t"
  print (Foo.metaclass.class_variables.include?("@@#{var}") ? Foo.metaclass.send(:class_variable_get, "@@#{var}").to_s : "-") + "\n"
end