From: Josh Cheek <josh.cheek@...>
Date: 2011-01-16T23:02:16+09:00
Subject: Current class

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Going through Metaprogramming Ruby, and the author, Paolo Perrotta points
out something I never paid attention to before.

class MyClass
  def method_one
    def method_two
      "hello!"
    end
  end
end

obj = MyClass.new
obj.method_one
obj.method_two # => "Hello!"

# my addition to show it is on MyClass, not obj's singleton class
MyClass.new.method_two # => "hello!"


He uses this example to show that Ruby keeps track of the current class. The
definition of method_two needs to know that it is being defined on, so even
though it is being executed in an instance of MyClass, it is defined as an
instance method of MyClass itself. This, understanding that Ruby keeps track
of not only self, but also the current class, is then used to explain the
difference between class_eval and instance_eval.



My questions are:
1. Is any other situation where this information is relevant?
2. Is there a way to access the current class? In the example below, you can
see that both instance_eval and class_eval set self to the class, but I
can't ask for the current class (the place methods would be defined on), I
only know how to ask for self's class.

class MyClass
end

MyClass.instance_eval do
  self        # => MyClass
  self.class  # => Class
  def current_class_is_singleton; end
end

MyClass.class_eval do
  self        # => MyClass
  self.class  # => Class
  def current_class_is_self; end
end

MyClass.instance_methods(false) # => [:current_class_is_self]
MyClass.singleton_methods       # => [:current_class_is_singleton]

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