（class_eval，define_method）vs（instance_eval，define_method）的Ruby单例方法

Question

Metaprogramming in ruby is great because I constantly use it to emulate prototype based programming and quickly write prototype solutions to some problems to test their viability. So I would like to know if there is any essential difference between the following pieces of code:

(class << some_object; self; end).class_eval do
  define_method(:method_name) do ... method body ... end
end

and

(class << some_object; self; end).instance_eval do
  define_method(:method_name) do ... method body ... end
end

Both versions of the code define a singleton method and I haven't yet come up against anything that has forced me to choose the (instance_eval, define_method) combination over the (class_eval, define_method) combination to define a singleton method and I'm wondering if there is some essential difference between the two.

Answer 1

No difference for define_method . But there is a difference when you use def .

o = Object.new  

# class_eval example
class << o; self; end.class_eval { def test1;  :test1; end  }
o.test1 #=> test1

# instance_eval example
class << o; self; end.instance_eval { def test2; :test2; end }
o.test2 #=> NoMethodError

Why the difference in behaviour between def and define_method ? define_method is a method call and so operates on the self in the eval context. The self in both instance_eval and class_eval is the same - it is the receiver (the eigenclass of o).

However def behaves differently, it does not operate on the self but instead on the default define . In the case of class_eval the default definee is the same as self but for instance_eval it is instead the metaclass of self .

How do we access the test2 method defined above? test2 must be an instance method defined on the metaclass of the eigenclass of o.

It is a class method on the eigenclass of o:

class << o; test2; end #=> :test2