Can someone please explain class << self to me?

Question

I'm jumping into rails programming for the first time and while looking at the code for some libraries I've downloaded, I occasionally notice the code:

class << self
  def func
     stuff
  end
end

I've tried searching the web for an explanation, but the << gets stripped from most useful search engines, so it ends up just searching for class self, which isn't very useful. Any insight would be appreciated.

Answer 1

In Ruby, class << foo opens up the singleton class of the object referenced by foo. In Ruby, every object has a singleton class associated with it which only has a single instance. This singleton class holds object-specific behavior, i.e. singleton methods.

So, class << self opens up the singleton class of self. What exactly self is, depends on the context you are in, of course. In a module or class definition body, it is the module or class itself, for example.

If all you are using the singleton class for, is defining singleton methods, there is actually a shortcut for that: def foo.bar.

Here's an example of how to use singleton methods to provide some "procedures" which don't really have any association with a particular instance:

class << (Util = Object.new)
  def do_something(n)
    # ...
  end
end

Util.do_something(n)