Object#dup and Object#clone in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Difference in frozen state after dup and clone

What is the output of the following Ruby code?

Ruby

str = "hello"
frozen_str = str.freeze
copy_dup = frozen_str.dup
copy_clone = frozen_str.clone
puts copy_dup.frozen?
puts copy_clone.frozen?

A

true
false

B

false
true

C

true
true

D

false
false

💡 Hint

Remember that dup does not copy the frozen state, but clone does.

In Ruby, dup creates a shallow copy but does not copy the frozen state of the original object. clone, however, copies the frozen state as well. So copy_dup.frozen? returns false while copy_clone.frozen? returns true.

❓ Predict Output

intermediate

Effect of modifying singleton methods after dup and clone

What will be the output of this Ruby code?

Ruby

obj = Object.new
def obj.greet; "hello"; end
obj_dup = obj.dup
obj_clone = obj.clone
puts obj_dup.respond_to?(:greet)
puts obj_clone.respond_to?(:greet)

A

true
true

B

false
false

C

false
true

D

true
false

💡 Hint

Singleton methods are copied by clone but not by dup.

Singleton methods belong to a single object. When you use dup, singleton methods are not copied. When you use clone, singleton methods are copied. So obj_dup does not respond to greet, but obj_clone does.

🔧 Debug

advanced

Why does modifying a duped object affect the original?

Consider this Ruby code snippet. Why does modifying the duped object also change the original object?

Ruby

class Person
  attr_accessor :name, :address
end

person1 = Person.new
person1.name = "Alice"
person1.address = { city: "NY" }
person2 = person1.dup
person2.address[:city] = "LA"
puts person1.address[:city]

A Because dup creates a shallow copy, so nested objects like the address hash are shared.

B Because dup creates a deep copy, so changes to person2 do not affect person1.

C Because the address hash is frozen and cannot be changed.

D Because the name attribute is not copied by dup.

💡 Hint

Think about how dup copies nested objects.

The dup method creates a shallow copy of the object. This means that the top-level object is copied, but nested objects like the address hash are not duplicated; both objects share the same nested hash. So modifying the nested hash in person2 affects person1 as well.

📝 Syntax

advanced

Which code raises an error when cloning a frozen object?

Which of the following Ruby code snippets will raise a RuntimeError when cloning a frozen object?

A

obj = Object.new.freeze
obj.clone

B

obj = "text".freeze
obj.dup

C

obj = "text".freeze
obj.clone

D

obj = Object.new.freeze
obj.dup

💡 Hint

Cloning a frozen object of some classes raises an error, but dup usually does not.

Cloning a frozen String does not raise an error, but cloning a frozen Object raises a RuntimeError. Duping a frozen object does not raise an error in either case. So option A raises an error.

🚀 Application

expert

How to create a deep copy of an object with nested mutable state?

Given a Ruby object with nested mutable objects, which approach correctly creates a deep copy so that modifying nested objects in the copy does not affect the original?

A Use obj.clone to copy the object and its nested objects.

B Use obj.dup to copy the object and its nested objects.

C Use obj.freeze before duplicating to prevent changes.

D Use Marshal.load(Marshal.dump(obj)) to serialize and deserialize the object.

💡 Hint

Think about how to copy nested objects deeply in Ruby.

Using Marshal.load(Marshal.dump(obj)) serializes the object and then deserializes it, creating a deep copy including nested mutable objects. Both dup and clone create shallow copies, so nested objects remain shared. Freezing prevents modification but does not copy.