Ruby Classes within Classes (or Modules within Modules)

Question

As I'm reading different Ruby books, I've noticed that Ruby classes can be defined within other Ruby classes or modules. Here's an example of a class within a class:

class Outerclass   def foobar     puts "FOOBAR"   end    class Innerclass        def barfoo       puts "BARFOO"     end   end end 

Here's some code that I ran in IRB to try to understand this conceptually:

oc = Outerclass.new # => #<Outerclass:0x00000100a46478>  Outerclass.instance_methods(false) # => [:foobar]  ic = Outerclass::Innerclass.new # => #<Outerclass::Innerclass:0x00000100a0b120>  ic = Outerclass::Innerclass.instance_methods(false) # => [:barfoo] 

Here are my questions:

1. When the ruby interpreter first encounters my Class definition code above, does it go through the methods I've written and store it somewhere? I know that the instance method "foobar" doesn't actually get run since there's no call being made to it within the Outerclass definition.

2. Piggybacking off the 1st question, what about when Ruby encounters the class Innerclass? What happens here?

3. In general, what are some reasons why you would want to have classes within classes? Are there any advantages to doing this?

4. Does an instance of Outerclass know anything about the class Innerclass?

5. Does an instance of Innerclass know anything about the class Outerclass?

Appreciate any help you can provide!

Answer 1

When the interpreter is going through this file, it is assigning classes like this:

OuterClass = Class.new do    def foobar     puts "FOOBAR"   end    InnerClass = Class.new do        def barfoo       puts "BARFOO"     end   end end 

So when Ruby encounters the nested class, it assigns it to constant InnerClass which is assigned to constant OuterClass They have no relation to each other whatsoever.

The InnerClass has no inheritance to OuterClass:

  InnerClass.ancestors   => [InnerClass, Object, Kernel, BasicObject] 

When you call OuterClass::InnerClass constant you are refering to the InnerClass constant that is namespaced under the OuterClass contstant which equals the Class.new statement assigned to it.

A good book to read about this is "Metaprogramming Ruby". It goes into the details of classes, singletons, modules etc.