Inherited hook in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of inherited hook with class variable

What is the output of this Ruby code using the inherited hook and a class variable?

Ruby

class Parent
  @@count = 0
  def self.inherited(subclass)
    @@count += 1
    puts "Inherited called: #{@@count}"
  end
end

class Child1 < Parent; end
class Child2 < Parent; end

A

Inherited called: 1
Inherited called: 2

B

Inherited called: 1
Inherited called: 1

C

Inherited called: 2
Inherited called: 3

D No output

💡 Hint

Remember that class variables are shared among the class hierarchy.

The inherited hook is called each time a subclass is created. The class variable @@count is shared, so it increments with each subclass.

❓ Predict Output

intermediate

Value of class instance variable in inherited hook

What will be printed when the following Ruby code runs?

Ruby

class Base
  @count = 0
  def self.inherited(subclass)
    @count ||= 0
    @count += 1
    puts "Subclasses: #{@count}"
  end
end

class Sub1 < Base; end
class Sub2 < Base; end

A

Subclasses: 0
Subclasses: 1

B No output

C

Subclasses: 1
Subclasses: 2

D

Subclasses: 1
Subclasses: 1

💡 Hint

Class instance variables are stored on the class object and persist across multiple method calls.

The @count variable belongs to the Base class object and persists between calls to inherited, which is invoked on Base for each direct subclass. It increments from 1 to 2.

🔧 Debug

advanced

Why does this inherited hook not count subclasses correctly?

This code is intended to count how many subclasses inherit from Parent, but it raises a NoMethodError. Why?

Ruby

class Parent
  def self.inherited(subclass)
    @count += 1
    puts "Count: #{@count}"
  end
end

class ChildA < Parent; end
class ChildB < Parent; end

A Because @count is a class instance variable and is nil initially, so @count += 1 raises an error.

B Because @count is not initialized before incrementing, so it is nil and nil + 1 causes an error.

C Because @count is a class instance variable of Parent and is not shared with subclasses, but inherited is called on subclasses, so @count is nil each time.

D Because inherited hook is not called for subclasses, so @count never increments.

💡 Hint

Check the initial value of @count before incrementing.

The @count variable is nil initially. Trying to add 1 to nil causes a NoMethodError. It must be initialized before incrementing.

❓ Predict Output

advanced

Effect of inherited hook on subclass method

What is the output of this Ruby code that uses the inherited hook to add a method to subclasses?

Ruby

class Parent
  def self.inherited(subclass)
    subclass.define_singleton_method(:greet) do
      "Hello from #{subclass}"
    end
  end
end

class Child < Parent; end
puts Child.greet

A

RuntimeError: wrong number of arguments (given 0, expected 1)

B

NoMethodError: undefined method `greet' for Child:Class

C

Hello from Parent

D

Hello from Child

💡 Hint

The inherited hook adds a class method to the subclass.

The inherited hook defines a singleton method greet on the subclass, so calling Child.greet returns the expected string.

🧠 Conceptual

expert

Understanding inherited hook behavior with multiple levels

Consider this Ruby code with multiple inheritance levels. How many times is the inherited hook called and what is printed?

Ruby

class A
  def self.inherited(subclass)
    puts "Inherited: #{subclass}"
  end
end

class B < A; end
class C < B; end

A

Inherited: B
Inherited: C

B

Inherited: C

C

Inherited: B

D No output

💡 Hint

The inherited hook is called for every direct subclass.

The inherited hook in A is called when B inherits from A, and again when C inherits from B (because B inherits from A, so B's inherited hook is inherited from A).

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