How does one - without inheritance - override a class method and call the original from within the new method?

I found one source which successfully overrode Time.strftime like this:

class Time
  alias :old_strftime :strftime
  def strftime
    #do something
    old_strftime
  end
end

The trouble is, strftime is an instance method. I need to override Time.now - a class method - in such away that any caller gets my new method, while the new method still calls the original .now method. I've looked at alias_method and have met with no success.

From stackoverflow

• Class methods are just methods. I highly recommend against this, but you have two equivalent choices:

  class Time
    class << self
      alias_method :old_time_now, :now
  
      def now
        my_now = old_time_now
        # new code
        my_now
      end
    end
  end
  
  class << Time
    alias_method :old_time_now, :now
  
    def now
      my_now = old_time_now
      # new code
      my_now
    end
  end
  

• This is kinda hard to get your head around sometimes, but you need to open the "eigenclass" which is the singleton associated with a specific class object. the syntax for this is class << self do...end.

  class Time
    alias :old_strftime :strftime
  
    def strftime
      puts "got here"
      old_strftime
    end
  end
  
  class Time
    class << self
      alias :old_now :now
      def now
        puts "got here too"
        old_now
      end
    end
  end
  
  t = Time.now
  puts t.strftime
  

• If the you need to override it for testing purposes (the reason I normally want to override Time.now), Ruby mocking/stubbing frameworks will do this for you easily. For instance, with RSpec (which uses flexmock):

  Time.stub!(:now).and_return(Time.mktime(1970,1,1))
  

  By the way, I highly recommend avoiding the need to stub out Time.now by giving your classes an overrideable clock:

  class Foo
    def initialize(clock=Time)
      @clock = clock
    end
  
    def do_something
      time = @clock.now
      # ...
    end
  end
  

• I've been trying to figure out how to override an instance method using modules.

  module Mo
    def self.included(base)
      base.instance_eval do
        alias :old_time_now :now
        def now
          my_now = old_time_now
          puts 'overrided now'
          # new code
          my_now
        end
      end
    end
  end
  Time.send(:include, Mo) unless Time.include?(Mo)
  
  > Time.now
  overrided now
  => Mon Aug 02 23:12:31 -0500 2010