classattr.rb

class MyClass
  @@myvar = "My Var"
  def self.myvar
    @@myvar
  end

  def self.myvar=(new_value)
    @@myvar = new_value
  end

  def foo
    puts 'foo'
  end
end

o = MyClass.new
o.foo
puts o.class.myvar
o.class.myvar = "new value"
puts o.class.myvar