From: gwtmp01@...
Date: 2006-07-15T03:05:59+09:00
Subject: Re: Don't understand :: in ActionController::Base


On Jul 14, 2006, at 9:13 AM, P�l Bergstr�m wrote:
> I understand the concept of classes and subclasses thanks to the
> excellent "Programming Ruby - The Pragmatic Programmer's Guide".
> However, I don't understand what :: signify in terms of inheritance,
> parent and child in e.g.

This seems to be a common source of confusion when learning Ruby
(myself included).

It is really pretty simple.  Class and Module objects are referenced
via hierarchical names:

	A
	A::B
	ActionController::Base
	ActiveRecord::Base
	Process::Sys

The confusion arises when you assume that the names imply class/subclass
relationships. The hierarchy of *names* is 100% independent of the
hierarchy of classes.

The scope operator (::) provides the syntax for constructing the
hierarchical class names but has no impact on the inheritance
structure of the classes:

class A
   class A1; end
end

class B
   class B1; end
end

class C < A; end
class D < A::A1; end
class E < B::B1; end
class B::B1::B2 < A::A1; end
class A::A1::A2 < B::B1::B2; end

 From those definitions you get the following inheritance structure:

Object  + -- A --------- C
         |
         + -- A::A1 --+-- D
         |            |
         |            +-- B::B1::B2 -- A::A1::A2
         |
         + -- B
         |
         + -- B::B1 ----- E


The associated naming hierarchy is

top + -- A -- A1 -- A2
     |
     + -- B -- B1 -- B2
     |
     + -- C
     |
     + -- D
     |
     + -- E


Gary Wright