Ruby class-level instance variable

Question

So I was doing a refresher on Ruby and I saw this guy's blog about creating class-level instance variable in Ruby. I am still trying to understand what the code actually does here. His blog can be found here

http://railstips.org/blog/archives/2006/11/18/class-and-instance-variables-in-ruby/

and I have created a simple code based on his example to show what I am trying to understand

class Polygon
  class << self; attr_accessor :sides end
  @sides = 10
  def initialize
  end
end

class Triangle < Polygon
  @sides = 3
  class << self; attr_accessor :sides end
  def initialize
  end
end

puts Triangle.sides #3
puts Polygon.sides #10

So the line that I really want to understand is (probably you guys have guessed it),

class << self; attr_accessor :sides end

What does this really do? what is he appending self to class ? is class an array then? Please elaborate as much as you can. Thank you.

Answer 1

The << is not a method (that is not exclusive to Array BTW), but is the syntax for defining a metaclass

Basically, a metaclass is THE class of a single object (some people calls them singleton classes). For example, if you define

x = Foo.new
y = Foo.new
class << x
  def quack
    "Quack!"
  end
end

then calling x.quack will return "Quack", but y.quack will throw a NoMethodError. So, the code is only evaluated on x's metaclass.

But... classes are objects too, right? So, when you evaluate that line, it is the equivalent of doing

class << Triangle
  attr_accessor :sites
end

which will just define an instance variable in the metaclass of Triangle. This is, the Triange class, which is an object too, will have an instance variable called sides

More info in this and this links. Once you get the idea, go to the nearest irb console and experiment with that.