cattr_accessor & attr_accessor

This defines attr_accessors at a class level instead of instance level.

  class Foo
    cattr_accessor :greeting
  end

  Foo.greeting = "Hello"

This could be compared to, but is not the same as doing this:

  class Bar
    class << self
      attr_accessor :greeting
    end
  end

  Bar.greeting = "Hello"

The difference might not be apparent at first, but cattr_accessor will make the accessor inherited to the instances:

  Foo.new.greeting #=> "Hello"
  Bar.new.greeting # NoMethodError: undefined method `greeting' for #

This inheritance is also not copy-on-write in case you assumed that:

  Foo.greeting  #=> "Hello"
  foo1, foo2 = Foo.new, Foo.new

  foo1.greeting = "Hi!"

  Foo.greeting  #=> "Hi!"
  foo2.greeting #=> "Hi!"

This makes it possible to share common state (queues, semaphores, etc.), configuration (max value, etc.) or temporary values through this.