Wrapping an instance method

gistfile1.rb

module Instrument
  def instrument(method)
    alias_method "#{method}_old", "#{method}"

    define_method method do |*args|
      t = Time.now.to_i
      send("#{method}_old", *args)
      result = Time.now.to_f - t
      p "#{method}: #{result} seconds"
    end
  end
end

class Calculator
  extend Instrument

  def five_times(n)
    p 5 * n
  end
  instrument :five_times
end

Calculator.new.five_times(5)