Object#dup and Object#clone in Ruby - Mini Project: Build & Apply

Understanding Object#dup and Object#clone in Ruby

📖 Scenario: Imagine you have a simple Ruby object representing a book in a library system. You want to create copies of this book object to make changes without affecting the original.

🎯 Goal: You will learn how to create copies of an object using dup and clone methods in Ruby, and observe the differences between them.

📋 What You'll Learn

Create a Ruby class called Book with attributes title and author

Create an instance of Book with specific values

Create copies of the book object using dup and clone

Print the original and copied objects to compare

💡 Why This Matters

🌍 Real World

Copying objects is useful when you want to work with similar data without changing the original, like duplicating a book record to edit details safely.

💼 Career

Understanding object copying helps in Ruby programming for tasks like caching, undo features, or managing state without side effects.

1

Create the Book class and an instance

Create a Ruby class called Book with an initialize method that takes title and author as parameters and sets them as instance variables. Then create an instance called original_book with title set to "The Ruby Way" and author set to "Hal Fulton".

Ruby

# Define the Book class and create original_book instance
# Your code here

Hint

Use attr_accessor for title and author. Initialize instance variables in initialize. Create original_book with given values.

2

Create copies using dup and clone

Create two new variables: dup_book by calling original_book.dup and clone_book by calling original_book.clone.

Ruby

class Book
  attr_accessor :title, :author

  def initialize(title, author)
    @title = title
    @author = author
  end
end

original_book = Book.new("The Ruby Way", "Hal Fulton")
# Create dup_book and clone_book from original_book
# Your code here

Hint

Use dup and clone methods on original_book to create copies.

3

Modify the copies' attributes

Change the title of dup_book to "The Ruby Way - Dup" and the title of clone_book to "The Ruby Way - Clone".

Ruby

class Book
  attr_accessor :title, :author

  def initialize(title, author)
    @title = title
    @author = author
  end
end

original_book = Book.new("The Ruby Way", "Hal Fulton")
dup_book = original_book.dup
clone_book = original_book.clone
# Change titles of dup_book and clone_book
# Your code here

Hint

Assign new strings to the title attribute of dup_book and clone_book.

4

Print the titles of all books

Print the title of original_book, dup_book, and clone_book each on a new line using puts.

Ruby

class Book
  attr_accessor :title, :author

  def initialize(title, author)
    @title = title
    @author = author
  end
end

original_book = Book.new("The Ruby Way", "Hal Fulton")
dup_book = original_book.dup
clone_book = original_book.clone
dup_book.title = "The Ruby Way - Dup"
clone_book.title = "The Ruby Way - Clone"
# Print titles of original_book, dup_book, and clone_book
# Your code here

Hint

Use puts to print the title of each book object on separate lines.

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