obj.instance_eval and mod.module_eval(mod.class_eval) usage

class C
  def initialize
    @x = 1
  end
end

c = C.new
c.instance_eval { puts @x }
puts c.instance_variable_get "@x"

C.class_eval do
  def some_method
    puts "Created in class_eval"
  end
end
c = C.new
c.some_method