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.

Practice

1. What is the main difference between dup and clone in Ruby?

easy

A. clone copies frozen state and singleton methods; dup does not.

B. dup copies frozen state and singleton methods; clone does not.

C. dup creates a deep copy; clone creates a shallow copy.

D. clone only copies instance variables; dup copies the entire object.

Solution

1. Step 1: Understand what dup copies

   dup creates a shallow copy but does not copy frozen state or singleton methods.

2. Step 2: Understand what clone copies

   clone copies everything including frozen state and singleton methods.

3. Final Answer:

   clone copies frozen state and singleton methods; dup does not. -> Option A

4. Quick Check:

   clone copies all [OK]

Hint: Remember: clone copies everything, dup skips frozen and singleton [OK]

Common Mistakes:

• Thinking dup copies frozen state
• Confusing shallow and deep copy
• Assuming clone skips singleton methods

2. Which of the following is the correct syntax to create a copy of an object obj using dup?

easy

A. copy = obj.dup()

B. copy = dup(obj)

C. copy = obj.clone()

D. copy = obj.copy()

Solution

1. Step 1: Identify the method call syntax

   In Ruby, dup is called on the object as obj.dup or obj.dup().

2. Step 2: Check other options

   clone is a different method, dup(obj) and copy() are invalid.

3. Final Answer:

   copy = obj.dup() -> Option A

4. Quick Check:

   Use obj.dup() to copy [OK]

Hint: Call dup as a method on the object: obj.dup() [OK]

Common Mistakes:

• Using clone instead of dup
• Trying dup(obj) as a function
• Using non-existent copy() method

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

class MyClass
  def greet
    "Hello"
  end
end

obj = MyClass.new
obj_singleton = obj.clone

def obj_singleton.greet
  "Hi"
end

copy1 = obj.dup
copy2 = obj.clone

puts copy1.greet
puts copy2.greet

medium

A. Error at runtime

B. Hello
Hi

C. Hi
Hi

D. Hello
Hello

Solution

1. Step 1: Understand singleton method copying

   Singleton methods are copied by clone but not by dup. Here, obj_singleton has a singleton method greet returning "Hi".

2. Step 2: Check what copy1 and copy2 get

   copy1 = obj.dup does not copy singleton methods, so copy1.greet calls original method returning "Hello".
   copy2 = obj.clone copies singleton methods only from obj, not from obj_singleton. Since obj has no singleton method, copy2.greet returns "Hello".

3. Final Answer:

   Hello Hello -> Option D

4. Quick Check:

   dup and clone copy singleton only from original object [OK]

Hint: Singleton methods copy only from original object, not clones [OK]

Common Mistakes:

• Assuming clone copies singleton methods from clones
• Thinking dup copies singleton methods
• Expecting 'Hi' output from copy2

4. Consider this Ruby code snippet:

str = "hello"
str.freeze
copy = str.dup
puts copy.frozen?

What is the output and why? If there is an error, identify and fix it.

medium

A. true

B. false

C. NoMethodError

D. RuntimeError

Solution

1. Step 1: Understand freezing and dup behavior

   The string str is frozen. When dup is called, it copies the object but does NOT copy the frozen state.

2. Step 2: Check frozen state of copy

   Since dup skips frozen state, copy.frozen? returns false.

3. Final Answer:

   false -> Option B

4. Quick Check:

   dup skips frozen state, so frozen? = false [OK]

Hint: dup skips frozen state, clone keeps it [OK]

Common Mistakes:

• Expecting copy to be frozen
• Confusing frozen? method with freeze
• Assuming dup raises error on frozen objects

5. You have an object obj with singleton methods and it is frozen. You want to create a copy that preserves both the singleton methods and the frozen state. Which method should you use and why?

hard

A. Use obj.dup because it copies singleton methods but not frozen state.

B. Use obj.dup and then manually freeze the copy.

C. Use obj.clone because it copies both singleton methods and frozen state.

D. Use obj.clone and then manually add singleton methods.

Solution

1. Step 1: Identify method that copies singleton methods and frozen state

   dup skips frozen state and singleton methods, so it is not suitable.

2. Step 2: Confirm clone behavior

   clone copies both singleton methods and frozen state automatically.

3. Final Answer:

   Use obj.clone because it copies both singleton methods and frozen state. -> Option C

4. Quick Check:

   clone copies all including frozen and singleton [OK]

Hint: clone copies frozen state and singleton methods together [OK]

Common Mistakes:

• Using dup and expecting frozen state copied
• Manually adding singleton methods after clone
• Freezing after dup instead of cloning