From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2013-04-12T03:14:47+09:00
Subject: Re: confusion with Struct class

On Thu, Apr 11, 2013 at 7:38 PM, Love U Ruby <lists@ruby-forum.com> wrote:
> Here is another confusion:
>
> I tried the below code:
>
> ===============================
> Customer = Struct.new(:name, :address) do |x|
>   def x.greeting
>     "Hello #{name}!"
>   end
> end
>
> p Customer.greeting #=> "Hello Customer!"
> ===============================
>
> Now my question is - How does instance variable has been assigned to
> `Customer`?

The object x that is passed into the block is the class that is being
created by Struct. When you define x.greeting, this is a singleton
method on the class object, and when inside that method you call
"name", you are actually calling a method of the x object, which is
class. With this test, you have discovered that classes have a "name"
method, that contains the constant to which they are assigned (this is
done either when doing class X, or X = <a class>):

1.9.2p290 :008 > Customer.methods
 => [...:name,...]

As you can see the class Customer has a name method.

If you create a class without assigning it to a constant, it doesn't
have a name:

1.9.2p290 :009 > c = Class.new
 => #<Class:0x00000002519990>
1.9.2p290 :010 > c.name
 => nil

on the other hand:

1.9.2p290 :011 > C = Class.new
 => C
1.9.2p290 :012 > C.name
 => "C"

and

1.9.2p290 :013 > class D
1.9.2p290 :014?>   end
 => nil
1.9.2p290 :015 > D.name
 => "D"

Hope this helps,

Jesus.