How to understand the difference between class_eval() and instance_eval()?

Question

Foo = Class.new
Foo.class_eval do
  def class_bar
    \"class_bar\"
  end
end
Foo.instance_eval do
  def instance_bar
    \"instance_bar\"
  end
end
Foo.class_bar            


        

Answer 1

I think you got it wrong. class_eval adds the method in the class, so all instances will have the method. instance_eval will add the method just to one specific object.

foo = Foo.new
foo.instance_eval do
  def instance_bar
    "instance_bar"
  end
end

foo.instance_bar      #=> "instance_bar"
baz = Foo.new
baz.instance_bar      #=> undefined method