Last week I discussed the differences between class_eval and instance_eval.
I stated there were two basic rules to follow when choosing which method to use:
- When the object is a class use class_eval, you will typically be using the def keyword
- When the object is an instance use instance_eval
When we use instance_eval to define methods where are the located? The are stored in the instance’s singleton class. I will write more about Ruby’s singleton class in the future, but for now you may want to read Ola Bini’s excellent post on the subject.
Let’s look at an example:
class MyClass; end my_instance = MyClass.new my_instance.instance_eval do # add the method to the instance's singleton class def foo "bar" end end puts my_instance.foo # => "bar" another_instance = MyClass.new begin # since the method exists only on the my_instance singleton class, calling # it on another_instance will raise a NoMethodError exception puts another_instance.greeting rescue NoMethodError puts "The foo method does not exist" endWhat happens when we call instance_eval on Class?
MyClass.instance_eval do def baz "qux" end end puts MyClass.baz # => "qux"
As you can see it defines a class method. I would argue that you should never use instance_eval on a class (although my last post regarding the subject suggested you could). I feel for a beginner in Ruby it may confuse. Stick to class_eval for class objects, and instance_eval for instances. If you need to dynamically define a class method then use this technique:
MyClass.class_eval do class << self # define .quux as a class method # def quux "corge" end end def grault "garply" end end puts MyClass.quux # => "corge" puts my_instance.grault # => "garply"