How to Freeze String in Ruby: Syntax and Examples

In Ruby, you can freeze a string by calling the freeze method on it, which makes the string immutable. Alternatively, you can use the magic comment # frozen_string_literal: true at the top of your file to freeze all string literals automatically.

Syntax

To freeze a string in Ruby, use the freeze method on a string object. This prevents any further modifications to that string.

You can also enable frozen string literals for the entire file by adding the magic comment # frozen_string_literal: true at the top. This makes all string literals immutable by default.

ruby

my_string = "hello"
my_string.freeze

# or enable frozen string literals for the whole file
# frozen_string_literal: true

another_string = "world"

Example

This example shows how freezing a string prevents changes and how the frozen string literal comment works.

ruby

# Using freeze method
my_string = "hello"
my_string.freeze

begin
  my_string << " world"
rescue => e
  puts e.message
end

# Using frozen string literal comment
# frozen_string_literal: true

another_string = "ruby"
begin
  another_string << " rocks"
rescue => e
  puts e.message
end

Output

can't modify frozen String
can't modify frozen String

Common Pitfalls

A common mistake is trying to modify a frozen string, which raises a RuntimeError. Also, freezing a string does not affect copies or new strings created from it.

Remember that string literals are mutable by default unless you use the magic comment.

ruby

# Wrong way: modifying a frozen string
str = "hello".freeze
str << " world"  # Raises error

# Right way: duplicate before modifying
str2 = str.dup
str2 << " world"
puts str2  # Outputs: hello world

Output

hello world

Quick Reference

Action	Code Example	Effect
Freeze a string	"hello".freeze	Makes the string immutable
Enable frozen literals	# frozen_string_literal: true	Freezes all string literals in the file
Modify copy safely	str2 = str.dup	Creates a mutable copy to modify

Key Takeaways

Use freeze to make a string immutable and prevent changes.

Add # frozen_string_literal: true at the top of your file to freeze all string literals automatically.

Modifying a frozen string raises a RuntimeError, so duplicate it first if you need a mutable copy.

Freezing strings can improve performance by avoiding unnecessary object copies.

Remember that freezing affects the string object itself, not other strings created from it.