From: "Jan E." <lists@...>
Date: 2012-04-11T11:56:05+09:00
Subject: Re: A Beginner's Question about Metaprogramming

Phil Stone wrote in post #1055921:
> Thanks, but what does this whole line mean?:
>
> def self.metaclass; class << self; self; end; end
>
> If I am correct, this is defining a Class method named metaclass, which
> has a method named self...? Please elaborate.

No, "self" is not a method, it's a keyword, which refers to the current 
object (many other languages use "this" instead).

The metaclass method returns the eigenclass (aka meta class aka 
singleton class) of the Creature class. It does this by opening the 
eigenclass and using the "self" keyword inside the class body. The value 
of self (the eigenclass) then becomes the value of the whole class 
expression.

class << self  # open the eigenclass of self (self is the Creature 
class)
  self         # in this context, self refers to the eigenclass
end            # the value of this expression is the value of self

This is a pretty common trick. However, it isn't needed any longer, 
because Ruby 1.9 already has the method Object#singleton_class built-in.

In fact, you don't even have to get the singleton class. You can simply 
call Object#define_singleton_method.

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