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).