Included hook in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code using the included hook?

Consider this Ruby module and class. What will be printed when MyClass.new.greet is called?

Ruby

module Greeter
  def self.included(base)
    puts "Greeter included in #{base}"
  end

  def greet
    "Hello!"
  end
end

class MyClass
  include Greeter
end

puts MyClass.new.greet

A

Greeter included in MyClass
Hello!

B Hello!

C

Greeter included in Greeter
Hello!

D No output, only returns "Hello!"

💡 Hint

Remember that self.included runs when the module is included in a class.

The included hook runs when the module is included, printing the message with the class name. Then greet returns "Hello!" which is printed by puts.

❓ Predict Output

intermediate

What happens when including a module with an included hook that modifies the base class?

What will be the output of this Ruby code?

Ruby

module Tracker
  def self.included(base)
    base.class_eval do
      def tracked_method
        "Tracked method called"
      end
    end
  end
end

class Widget
  include Tracker
end

puts Widget.new.tracked_method

A SyntaxError due to class_eval usage

B NoMethodError: undefined method `tracked_method' for #<Widget:...>

C Tracked method called

D RuntimeError: included hook failed

💡 Hint

The included hook can add methods to the including class dynamically.

The included hook uses class_eval to add tracked_method to the class, so calling it works and prints the string.

❓ Predict Output

advanced

What is the output when multiple modules with included hooks are included?

Given these modules and class, what is printed when Example.new.test is called?

Ruby

module A
  def self.included(base)
    puts "Module A included"
  end
  def test
    "A"
  end
end

module B
  def self.included(base)
    puts "Module B included"
  end
  def test
    "B"
  end
end

class Example
  include A
  include B
end

puts Example.new.test

A

Module A included
Module B included
B

B

Module B included
Module A included
A

C

Module A included
Module B included
A

D

Module B included
Module A included
B

💡 Hint

Modules included later override methods of earlier modules.

Both included hooks run in order of inclusion, printing their messages. The last included module B overrides test, so calling test returns "B".

❓ Predict Output

advanced

What error occurs if included hook tries to call an undefined method on base?

What error will this Ruby code raise?

Ruby

module M
  def self.included(base)
    base.undefined_method
  end
end

class C
  include M
end

A SyntaxError: unexpected method call

B RuntimeError: included hook failed

C No error, code runs silently

D NoMethodError: undefined method `undefined_method' for C:Class

💡 Hint

Calling a method that does not exist on a class raises a NoMethodError.

The included hook tries to call undefined_method on the class C, which does not exist, causing a NoMethodError.

🧠 Conceptual

expert

How does Ruby's included hook differ from extend in modules?

Which statement best describes the difference between include with an included hook and extend in Ruby modules?

A <code>include</code> adds class methods and triggers <code>included</code> hook; <code>extend</code> adds instance methods without triggering <code>included</code>.

B <code>include</code> adds instance methods and triggers <code>included</code> hook; <code>extend</code> adds class methods without triggering <code>included</code>.

C <code>include</code> and <code>extend</code> both add instance methods but only <code>extend</code> triggers <code>included</code> hook.

D <code>include</code> adds instance methods without triggering <code>included</code>; <code>extend</code> adds class methods and triggers <code>included</code> hook.

💡 Hint

Think about what methods are added and when the included hook runs.

include adds instance methods to a class and triggers the included hook. extend adds methods as class methods and does not trigger included.