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