From: Robert Klemme <bob.news@...>
Date: 2005-08-10T00:16:09+09:00
Subject: Re: help me understand this ruby idiom: class << self; alias_method :x :y end

Kelly Felkins wrote:
> I'm trying to understand classes and objects. The Pick axe book
> points out the "once" method in class Date and how it is defined
> using the object singleton technique of "class << self". Actually
> Date uses this in several places. Up until seeing Date I thought the
> reason for this idiom was to add/replace methods in classes *when you
> don't have convenient access to the class definition* -- basically,
> use this when it's not your class you need to enhance. But the fact
> that Date uses it several times indicates there is another reason to
> use it.

As you guessed the reasons are different: typically you use "class
<<something...end" if you have to define several methods or if you need a
class context for things to work (for example "alias").

These are equivalent:

class Foo
  class <<self
    def m1() "foo" end
  end

  class <<Foo
    def m2() "foo" end
  end

  def self.m3() "foo" end
  def Foo.m4() "foo" end
end

class <<Foo
  def m5() "foo" end
end

def Foo.m6() "foo" end

x = Foo
def x.m7() "foo" end

class <<x
  def m8() "foo" end
end

Now you have methods m1 to m8 as instance methods of Foo.  You can do
Foo.m1(), Foo.m2 etc.

More technically, with class <<something ... end you make definition for
the so called "singleton class" the class instance responsible for the
single instance at hand (which happens to be a class object in this case;
but you can repeat the example above with any object instead of Foo).  You
cannot create instances from this class:

irb(main):001:0> class Foo
irb(main):002:1> p new
irb(main):003:1> end
#<Foo:0x10192d58>
=> nil
irb(main):004:0> class <<Foo
irb(main):005:1> p new
irb(main):006:1> end
TypeError: can't create instance of virtual class
        from (irb):5:in `new'
        from (irb):5

> Here are some examples from Date:
> class << self; alias_method :leap?, :gregorian_leap? end
> class << self; alias_method :new0, :new end
>
> I just noticed that every use of class << self in Date contains an
> alias_method.
>
> Please enlighten me.

Hope, I could help.

Kind regards

    robert