Using undef in Ruby

Dec 14, 2015

Defining methods in Ruby is pretty simple. Type def, a name for your method, then some code, and finally end it. It looks like this

def name

Maybe undefine isn’t the right word. I think hides is a better description. It hides the method.

Once you use undef in a class, you cannot call a method of that name.

Why Use undef

So how would you use undef?

The best case I can come up with is using it in metaprogramming. Maybe you want to make sure no one defines a certain method at runtime. Ruby can be pretty crazy. Remember, everything is open. Restricting a method of a certain name can be handy.

How to Use it

Using it is pretty straightforward. Let’s say we have a class called Name that takes a first and last name. But for the sake of this example, we don’t want anyone calling first_name, even though it’s totally legit and exists.

The class might look like this.

class Name
  def initialize(first_name, last_name)
    @first_name = first_name
    @last_name = last_name

  def first_name
    puts @first_name

  def last_name
    puts @last_name

  def full_name
    puts @first_name + “ “ + @last_name

  undef first_name

Then loading this in IRB and trying to call first_name will result in an error.

irb -r ./testing_undef.rb

@person =“Scott”, “Radcliff”)


You will see NoMethodError: undefined method first_name’ for #<Name:0x007fd51b1ca8e8>`

Because we called undef first_name after it was defined, we removed access to it. Even though we get a NoMethodError exception, it still exists. If we had called undef on a method that didn’t exist, that would have resulted in a NameError.

NameError: undefined method first_name’ for class Name'

While it’s possible to undefine a method in Ruby, I’m not exactly sure how helpful it is in day to day programming.