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

ruby

Solution

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

Problem

Is there any difference between ``` module Foo class Engine < Rails::Engine end end ``` and ``` module Foo class Engine < ::Rails::Engine end end ```

Original source