Ruby undef Keyword

last modified October 18, 2026

This tutorial explains how to use Ruby's undef keyword. It removes method definitions from the current class or module.

The undef keyword marks a method as undefined in the class or module where it appears. Instances of that class, or classes that include the module, then raise NoMethodError when they call the method. Inside a class body, undef targets instance methods; class methods must be undefined in the class's singleton class.

undef is useful when you need to restrict certain functionality. It works with both built-in and user-defined methods. The change is not permanent: reopening the class and defining the method again restores it.

Basic undef Example

This simple example demonstrates removing a method from a class. After using undef, calling the method raises an error.

basic_undef.rb

class Greeter
  def hello
    puts "Hello!"
  end
end

greeter = Greeter.new
greeter.hello  # Works fine

class Greeter
  undef hello
end

greeter.hello  # Raises NoMethodError

The hello method is first defined and works normally. After undef hello, calling it raises NoMethodError.

undef with Inherited Methods

undef can block methods inherited from parent classes. This example shows how to prevent method inheritance.

undef_inheritance.rb

class Parent
  def inherited_method
    puts "From parent class"
  end
end

class Child < Parent
  undef inherited_method
end

Child.new.inherited_method  # Raises NoMethodError
Parent.new.inherited_method # Still works

The child class blocks the inherited method while the parent keeps it. The undefined entry stops method lookup in Child, so the call raises NoMethodError.

undef Multiple Methods

Ruby allows removing multiple methods with a single undef statement. Separate method names with commas.

multiple_undef.rb

class MultiMethod
  def method1; end
  def method2; end
  def method3; end
end

class MultiMethod
  undef method1, method2
end

obj = MultiMethod.new
obj.method3  # Works
obj.method1  # Raises NoMethodError
obj.method2  # Raises NoMethodError

Three methods are defined initially. The undef statement removes two of them at once. The remaining method stays available.

undef with Built-in Methods

You can use undef to make inherited core methods unavailable in a class. This example disables the to_s method.

undef_builtin.rb

class CustomObject
  undef to_s
end

obj = CustomObject.new
puts obj  # Raises NoMethodError

The to_s method is removed from CustomObject. Any attempt to convert the object to a string, including puts, now fails. This is one reason to use undef with care.

undef in Modules

undef works in modules too. This example shows method removal before including the module.

module_undef.rb

module MyModule
  def module_method
    puts "From module"
  end
  
  undef module_method
end

class MyClass
  include MyModule
end

MyClass.new.module_method  # Raises NoMethodError

The method is defined then immediately removed in the module. Classes including the module won't have access to it. This pattern can control exposed methods.

undef vs remove_method

Ruby also has remove_method which differs from undef. This example shows the key difference.

undef_vs_remove.rb

class Parent
  def example
    puts "Parent implementation"
  end
end

class Child < Parent
  def example
    puts "Child implementation"
  end
  
  remove_method :example  # Removes Child's version, Parent's remains
  # undef example        # Blocks lookup and raises NoMethodError
end

Child.new.example  # Outputs "Parent implementation"

remove_method deletes only the method defined directly in Child, so Ruby continues up the ancestor chain and finds Parent#example. undef instead installs an undefined entry that stops lookup, so Child.new.example would raise NoMethodError even though the parent still defines the method.

undef_method behaves like undef but can be called programmatically. Use remove_method to fall back to an ancestor's implementation, and undef or undef_method to block the method entirely.

Dynamic undef with send

You can remove methods dynamically with undef_method. Because it is a private method of Module, send lets you call it with a computed method name at runtime.

dynamic_undef.rb

class DynamicClass
  def method1; end
  def method2; end
end

[:method1, :method2].each do |name|
  DynamicClass.send(:undef_method, name)
end

obj = DynamicClass.new
obj.respond_to?(:method1)  # => false
obj.respond_to?(:method2)  # => false

The example removes methods in a loop. undef_method is the programmatic counterpart of undef, and the methods become unavailable on instances of the class.

Source

Ruby Keywords Documentation

This tutorial covered Ruby's undef keyword with practical examples showing method removal in various contexts. Use it carefully, since it makes the method unavailable until it is defined again.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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