Frozen objects in Ruby

Introduction

Freezing an object in Ruby means making it unchangeable. This helps keep data safe from accidental changes.

When you want to make sure a configuration value never changes during the program.

When sharing data between parts of a program and want to avoid accidental edits.

When you want to improve performance by preventing unnecessary copies of objects.

When debugging to catch unexpected changes to important objects.

Syntax

Ruby

object.freeze

Calling freeze on an object makes it immutable.

If you try to change a frozen object, Ruby raises a FrozenError.

Examples

Freezing a string prevents adding more text to it.

Ruby

str = "hello"
str.freeze
str << " world"  # This will cause an error

Freezing an array stops changing its elements.

Ruby

arr = [1, 2, 3]
arr.freeze
arr[0] = 10  # This will cause an error

Freezing a number does nothing because numbers can't be changed anyway.

Ruby

num = 100
num.freeze
# Numbers are already immutable, so freezing has no effect

Sample Program

This program freezes a string and then tries to change it. Ruby raises an error, which we catch and print. Finally, it prints the original string.

Ruby

name = "Alice"
name.freeze

begin
  name << " Smith"
rescue => e
  puts "Error: #{e.message}"
end

puts name

Important Notes

Freezing only affects the object itself, not objects inside it. For example, freezing an array does not freeze the objects inside the array.

You can check if an object is frozen by calling object.frozen?.

Summary

Freezing makes objects unchangeable to protect data.

Trying to change a frozen object causes an error.

Use freeze to keep important data safe from accidental changes.

Practice

1. What does the freeze method do to an object in Ruby?

easy

A. Makes the object mutable

B. Deletes the object from memory

C. Duplicates the object

D. Prevents any modifications to the object

Solution

1. Step 1: Understand the purpose of freeze

   The freeze method is used to make an object immutable, meaning it cannot be changed after freezing.

2. Step 2: Identify the effect on the object

   Once frozen, any attempt to modify the object will raise an error, so it prevents modifications.

3. Final Answer:

   Prevents any modifications to the object -> Option D

4. Quick Check:

   freeze makes object immutable = Prevents modifications [OK]

Hint: Freeze means no changes allowed to the object [OK]

Common Mistakes:

• Thinking freeze deletes the object
• Confusing freeze with duplication
• Believing freeze makes object mutable

2. Which of the following is the correct way to freeze a string object str in Ruby?

easy

A. str.freeze()

B. freeze str

C. str.freeze = true

D. freeze(str)

Solution

1. Step 1: Recall Ruby method call syntax

   In Ruby, methods are called on objects using dot notation, e.g., object.method.

2. Step 2: Identify the correct freeze syntax

   The correct way to freeze an object is by calling freeze on it: str.freeze(). The other options are invalid syntax.

3. Final Answer:

   str.freeze() -> Option A

4. Quick Check:

   Method call syntax = str.freeze() [OK]

Hint: Use dot notation to call freeze: object.freeze() [OK]

Common Mistakes:

• Using freeze as a standalone keyword
• Assigning freeze as a property
• Calling freeze like a function without dot

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

arr = [1, 2, 3]
arr.freeze
arr << 4
puts arr.inspect

medium

A. RuntimeError: can't modify frozen Array

B. [1, 2, 3, 4]

C. [1, 2, 3]

D. SyntaxError

Solution

1. Step 1: Understand freezing the array

   The array arr is frozen with arr.freeze, so it cannot be changed.

2. Step 2: Analyze the modification attempt

   The code tries to add an element with arr << 4, which modifies the array. Since it's frozen, this raises a runtime error.

3. Final Answer:

   RuntimeError: can't modify frozen Array -> Option A

4. Quick Check:

   Modifying frozen object = RuntimeError [OK]

Hint: Modifying frozen objects causes runtime errors [OK]

Common Mistakes:

• Expecting the array to change
• Thinking freeze allows modifications
• Confusing runtime error with syntax error

4. Identify the error in this Ruby code snippet:

name = "Alice"
name.freeze
name[0] = 'M'
puts name

medium

A. No error, output is "Mlice"

B. RuntimeError: can't modify frozen String

C. SyntaxError due to invalid assignment

D. TypeError: wrong type of argument

Solution

1. Step 1: Recognize string freezing

   The string name is frozen, so it cannot be changed after name.freeze.

2. Step 2: Understand the attempted modification

   Trying to change the first character with name[0] = 'M' modifies the string, which is forbidden and raises a runtime error.

3. Final Answer:

   RuntimeError: can't modify frozen String -> Option B

4. Quick Check:

   Changing frozen string = RuntimeError [OK]

Hint: Changing frozen strings causes runtime errors [OK]

Common Mistakes:

• Assuming strings can be changed after freeze
• Confusing syntax error with runtime error
• Ignoring freeze effect on string elements

5. Given this Ruby code, what will be the output?

hash = {a: [1, 2], b: [3, 4]}
hash.freeze
hash[:a] << 5
puts hash[:a].inspect

What does this tell us about freezing objects?

hard

A. RuntimeError: can't modify frozen Hash

B. [1, 2] - freeze prevents any changes including nested objects

C. [1, 2, 5] - freeze prevents changing the hash keys but not the objects inside

D. SyntaxError due to invalid hash modification

Solution

1. Step 1: Understand freezing the hash

   The hash object is frozen, so its structure (keys and values) cannot be changed directly.

2. Step 2: Analyze modification of nested array

   The value for key :a is an array, which is not frozen. Modifying this array with << 5 is allowed.

3. Step 3: Observe the output

   The array now contains [1, 2, 5], showing that freezing the hash does not freeze nested objects.

4. Final Answer:

   [1, 2, 5] - freeze prevents changing the hash keys but not the objects inside -> Option C

5. Quick Check:

   Freeze is shallow = nested objects can change [OK]

Hint: Freeze is shallow; nested objects stay mutable unless frozen [OK]

Common Mistakes:

• Assuming freeze affects nested objects
• Expecting runtime error on nested modification
• Confusing hash freezing with deep freeze