Ruby's double colon (::) operator usage differences

Question

Is there any difference between

module Foo
  class Engine < Rails::Engine
  end
end

and

module Foo
  class Engine < ::Rails::Engine
  end
end

Answer 1

Constants in Ruby are nested like files and directories in filesystem. So, constants are uniquely identified by their paths.

To draw an analogy with the file system:

::Rails::Engine #is an absolute path to the constant.
# like /Rails/Engine in FS.

Rails::Engine #is a path relative to the current tree level.
# like ./Rails/Engine in FS.

Here is the illustration of possible error:

module Foo

  # We may not know about this in real big apps
  module Rails
    class Engine 
    end
  end

  class Engine1 < Rails::Engine
  end

  class Engine2 < ::Rails::Engine
  end
end

Foo::Engine1.superclass
 => Foo::Rails::Engine # not what we want

Foo::Engine2.superclass
 => Rails::Engine # correct