What is this line of Ruby code doing?

Question

Possible Duplicate:
class << self idiom in Ruby

I was wondering what this code is doing. I don't understated this one line. I know what attr_accessor is.

class User
  class << self; attr_accessor :base_uri end
  ....
....
...

Answer 1

You will see class << self used often in Ruby. The easiest way to understand what it does is to think of it as (almost) equivalent to this:

class Abc
   self.attr_accessor ...
end

Which is basically the same as this:

Abc.class.attr_accessor ...

But you really can't do either of those because:

> Abc.class.attr_accessor :blah
NoMethodError: private method `attr_accessor' called for Class:Class

So to get around it you'd have to do:

> Abc.class.send(:attr_accessor, :blah)

Or:

class Abc
  self.send(:attr_accessor, :blah)
end 

That's where the << self construct comes in since it gives you access to these private methods. You are basically operating directly in "self-space".

Similarly when you see:

class Abc
   class << self
     def foo
     end
   end
end

It's just the same as saying:

class Abc
   def self.foo
   end
end

Which will define a class method, just like the code in your question will define a class-level attr_accessor.

EDIT

As for a less complicated way - Ruby doesn't really by default have a method that is able to create class-level attr_accessors. That's why you have to use the << self trick.

But Rails does define a method that does something similar without having to use << self. In Rails you have cattr_accessor:

class Abc
   cattr_accessor :blah
end

Abc.blah = 123
Abc.blah
>> 123

Answer 2

This let define an instance variable in the class context.

See the code:

class User
  class << self; attr_accessor :base_uri end
  @base_uri = "aaa";
end

p User.base_uri # will output "aaa"