Is there any difference between
module Foo class Engine < Rails::Engine end end and
module Foo class Engine < ::Rails::Engine end end 
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2 Answers
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 
4 Comments
Kevin Bedell
Right. So if you were to call Rails::Engine from a module using the name
Module Foo, it may interpret it as Foo::Rails::Engine. I've seen errors like this at times and had to add the additional :: in front of an include.
Flexoid
It was taken from the book "Metaprogramming Ruby" by Paolo Perrotta. Very useful book to deepen the knowledge of the Ruby.
Matheus Moreira
Nice analogy. This becomes problematic when one has modules with the same name in different scopes, since Ruby seems to use the first/nearest one it can find.
Aaron K
A subtle point: if
Rails::Engine is defined outside of Foo and it is also not defined inside Foo then Foo::Engine1.superclass will be Rails::Engine as desired.Rails::Engine #is a path relative to the current tree level. # like ./Rails/Engine in FS. This is not quite true!
Let's have an example:
module M Y = 1 class M Y = 2 class M Y = 3 end class C Y = 4 puts M::Y end end end # => 3 module M Y = 1 class M Y = 2 class C Y = 4 puts M::Y end end end # => 2 module M Y = 1 class M Y = 2 class M Y = 4 puts M::Y end end end # => 4 So when you say M::Y ruby looks up for the closest defintion no matter if it is inside the current scope or outer scope or outer outer scope etc.
1 Comment
Dave Nolan
which Ruby is this? On 2.2.4 I get 3, 3, and 4 and many "top level constant already referenced warnings"