Inherited hook in Ruby - Mini Project: Build & Apply

Using the Inherited Hook in Ruby

📖 Scenario: Imagine you are creating a system to track different types of vehicles. You want to know whenever a new type of vehicle class is created that inherits from your main Vehicle class.

🎯 Goal: You will build a Ruby program that uses the inherited hook method to print a message every time a new subclass inherits from Vehicle.

📋 What You'll Learn

Create a class called Vehicle

Define an inherited method inside Vehicle that takes one argument called subclass

Inside the inherited method, print a message using puts that says: "New vehicle type created: "

Create two subclasses called Car and Truck that inherit from Vehicle

Print the messages when Car and Truck are defined

💡 Why This Matters

🌍 Real World

Using the <code>inherited</code> hook helps frameworks and libraries track subclasses automatically, such as in web frameworks or plugin systems.

💼 Career

Understanding hooks like <code>inherited</code> is useful for Ruby developers building extensible applications or working with metaprogramming.

1

Create the Vehicle class

Create a class called Vehicle with no content inside it.

Ruby

# Create a class called Vehicle
# Your code here

Hint

Use class Vehicle and end to define the class.

2

Add the inherited hook method

Inside the Vehicle class, define a method called self.inherited that takes one parameter called subclass. Inside this method, use puts to print "New vehicle type created: " followed by the subclass name using subclass.name.

Ruby

class Vehicle
  # Define the inherited hook method here
  # Your code here
end

Hint

The inherited method is a class method, so use self.inherited.

3

Create subclasses Car and Truck

Create two subclasses called Car and Truck that inherit from Vehicle. Use the syntax class Car < Vehicle and class Truck < Vehicle.

Ruby

class Vehicle
  def self.inherited(subclass)
    puts "New vehicle type created: #{subclass.name}"
  end
end

# Create subclass Car
# Your code here

# Create subclass Truck
# Your code here

Hint

Use class Car < Vehicle and class Truck < Vehicle to create subclasses.

4

Run the program to see output

Run the program and observe the output. It should print two lines: "New vehicle type created: Car" and "New vehicle type created: Truck".

Ruby

class Vehicle
  def self.inherited(subclass)
    puts "New vehicle type created: #{subclass.name}"
  end
end

class Car < Vehicle
end

class Truck < Vehicle
end

# Run the program to see output
# Your code here

Hint

Just run the program. The messages print automatically when subclasses are defined.

Practice

1. What does the inherited hook method do in Ruby?

easy

A. It is called automatically when a subclass is created.

B. It runs when an instance of a class is created.

C. It is used to define class variables.

D. It is called when a method is overridden.

Solution

1. Step 1: Understand the purpose of inherited

   The inherited method is a special hook in Ruby that triggers when a subclass is created from a parent class.

2. Step 2: Differentiate from other hooks

   It is not related to instance creation, method overriding, or variable definition, but specifically to subclass creation.

3. Final Answer:

   It is called automatically when a subclass is created. -> Option A

4. Quick Check:

   Inherited hook = called on subclass creation [OK]

Hint: Inherited hook triggers only on subclass creation [OK]

Common Mistakes:

• Confusing inherited with initialize method
• Thinking it runs on instance creation
• Mixing it with method override hooks

2. Which of the following is the correct way to define an inherited hook in a Ruby class?

easy

A. def self.inherited; puts "New subclass created"; end

B. def inherited(self); puts "New subclass created"; end

C. def inherited(subclass); puts "New subclass created"; end

D. def self.inherited(subclass); puts "New subclass created"; end

Solution

1. Step 1: Recognize the method signature

   The inherited hook must be defined as a class method, so it needs self.inherited and takes one argument: the subclass.

2. Step 2: Check method parameters and syntax

   def self.inherited(subclass); puts "New subclass created"; end correctly defines the class method with the subclass parameter. Incorrect versions lack self. (instance method), omit the subclass parameter, or use wrong parameters like self.

3. Final Answer:

   def self.inherited(subclass); puts "New subclass created"; end -> Option D

4. Quick Check:

   Class method with subclass param = def self.inherited(subclass); puts "New subclass created"; end [OK]

Hint: Inherited hook is a class method with subclass param [OK]

Common Mistakes:

• Defining inherited as instance method
• Omitting the subclass parameter
• Forgetting self. for class method

3. What will be the output of this Ruby code?

class Parent
  def self.inherited(subclass)
    puts "Subclass: #{subclass}"
  end
end

class Child < Parent
end

medium

A. No output

B. Subclass: Child

C. Subclass: Parent

D. SyntaxError

Solution

1. Step 1: Understand when inherited runs

   The inherited method in Parent runs automatically when a subclass is created.

2. Step 2: Identify subclass creation

   When Child < Parent is declared, Parent.inherited(Child) is called, printing "Subclass: Child".

3. Final Answer:

   Subclass: Child -> Option B

4. Quick Check:

   Subclass creation triggers inherited hook output [OK]

Hint: Inherited prints subclass name on subclass creation [OK]

Common Mistakes:

• Expecting output on instance creation
• Confusing subclass with parent class name
• Thinking no output occurs

4. Identify the error in this Ruby code using the inherited hook:

class Base
  def inherited(subclass)
    puts "Subclass created: #{subclass}"
  end
end

class Derived < Base
end

medium

A. The inherited method should be a class method with self.

B. The subclass parameter is missing.

C. The puts statement syntax is incorrect.

D. Derived class should not inherit from Base.

Solution

1. Step 1: Check method definition context

   The inherited hook must be a class method, so it requires def self.inherited(subclass).

2. Step 2: Identify the error in the code

   Here, inherited is defined as an instance method, missing self., so it won't be called on subclass creation.

3. Final Answer:

   The inherited method should be a class method with self. -> Option A

4. Quick Check:

   Inherited hook must be class method [OK]

Hint: Inherited must be defined with self. to work [OK]

Common Mistakes:

• Defining inherited as instance method
• Omitting self. in method definition
• Misunderstanding inherited hook usage

5. How can you use the inherited hook to automatically add a class variable @@subclasses that tracks all subclasses of a parent class?

hard

A. Use initialize method to track subclasses in @@subclasses.

B. Define an instance method inherited to add subclass to @@subclasses.

C. Define self.inherited(subclass) to append subclass to @@subclasses array.

D. Manually add subclasses to @@subclasses outside the class.

Solution

1. Step 1: Understand subclass tracking

   To track subclasses, use a class variable @@subclasses initialized as an array in the parent class.

2. Step 2: Use inherited hook to update tracking

   Define self.inherited(subclass) to append the new subclass to @@subclasses automatically when a subclass is created.

3. Final Answer:

   Define self.inherited(subclass) to append subclass to @@subclasses array. -> Option C

4. Quick Check:

   Inherited hook + class variable tracks subclasses [OK]

Hint: Use inherited hook to update subclass list automatically [OK]

Common Mistakes:

• Using instance methods to track subclasses
• Tracking subclasses in initialize method
• Forgetting to initialize @@subclasses as array