Undefining Methods in Ruby

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
 …code
end

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.

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

class Name
 def initialize(first_name, last_name)
   @first_name = first_name
   @last_name = last_name
 end
 def first_name
   puts @first_name
 end
 def last_name
   puts @last_name
 end
 def full_name
   puts @first_name + “ “ + @last_name
 end
 undef first_name
end
irb -r ./testing_undef.rb
@person = Name.new(“Scott”, “Radcliff”)
@person.first_name

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.