class-eval

I am a Ruby starter. I found both of these are quite similar (in output), but i couldn't understand the difference in the below context. For example, I have a class class Say def self.hello puts "hello" end end and can be extended like this class << Say def hi puts "hi" end end and also like this Say.class_eval do def self.bye puts "bye" end end When should I use << and when class_eval ? class_eval doesn't really have anything to do with class << className . A.class_eval do ... end is equivalent to class A ... end with a few differences. class_eval uses a block (or a string, but ignoring that

I am using class_eval to write code to be executed under the context of current class. In the following code, I want to add a counter for changes of attribute values. class Class def attr_count(attr_name) attr_name = attr_name.to_s attr_reader attr_name # create the attribute's getter class_eval %Q{ @count = 0 def #{attr_name}= (attr_name) @attr_name = attr_name @count += 1 end def #{attr_name} @attr_name end } end end class Foo attr_count :bar end f = Foo.new f.bar = 1 My understanding of class_eval is that it evaluates the block in the context of the runtime class - in my case, under class

I'm doing the SaaS Stanford class, trying to do Part 5 of this assignment I'm having a really hard time grasping this concept, this is what I've attempted to do: class Class def attr_accessor_with_history(attr_name) attr_name = attr_name.to_s attr_reader attr_name attr_reader attr_name + '_history' class_eval %Q'{def #{attr_name}(a);#{attr_name}_history.push(a) ; end;}' end end I'm probably doing all sorts of things wrong, read The Book Of Ruby chapter on metaprogramming and I still don't get it, can someone help me comprehend this? This was fun!!! class Class def attr_accessor_with_history