gist.rb

module InstanceExec
  Code = lambda do
    unless Object.new.respond_to?(:instance_exec)
      module InstanceExecHelper; end
      include InstanceExecHelper

      def instance_exec(*args, &block)
        begin
          old_critical, Thread.critical = Thread.critical, true
          n = 0
          n += 1 while respond_to?(mname="__instance_exec_#{ n }__")
          InstanceExecHelper.module_eval{ define_method(mname, &block) }
        ensure
          Thread.critical = old_critical
        end
        begin
          ret = send(mname, *args)
        ensure
          InstanceExecHelper.module_eval{ remove_method(mname) } rescue nil
        end
        ret
      end
    end
  end

  def InstanceExec.included(other)
    other.module_eval(&Code)
    super
  end

  def InstanceExec.extend_object(other)
    other.instance_eval(&Code)
    super
  end
end


if __FILE__ == $0
  class C
    include InstanceExec
    def foo(x) p x end
  end

  a = C.new
  b = Object.new
  def b.foo(x) p x end
  b.extend(InstanceExec)

  callback =
    lambda do |x|
      p self
      p x
    end

  a.instance_exec(42, &callback)
  b.instance_exec(42, &callback)
end