EC
FrozenError
RubyERRORNotableMutation

Attempt to modify a frozen object

Quick Answer

Call .dup or .clone to get a mutable copy of a frozen object before modifying it.

What this means

Raised when code attempts to modify an object that has been frozen with Object#freeze. FrozenError is a subclass of RuntimeError. String literals are frozen by default when frozen_string_literal: true is set.

Why it happens
  1. 1Modifying a string literal when frozen_string_literal: true is enabled
  2. 2Calling mutating methods (<<, gsub!, push) on an explicitly frozen object
  3. 3Modifying a frozen Hash or Array in a shared/constant context

Fix

Duplicate before mutating

Duplicate before mutating
CONSTANT = 'hello'.freeze

def shout(str)
  str.dup.upcase!   # dup gives a mutable copy
end

shout(CONSTANT)   # safe

Why this works

dup creates an unfrozen shallow copy, allowing mutation without affecting the original frozen object.

Code examples
Reproducing the errorruby
str = 'hello'.freeze
str << ' world'
# FrozenError: can't modify frozen String: "hello"
frozen_string_literal magic commentruby
# frozen_string_literal: true
str = 'hello'
str << '!'   # FrozenError
Using dup to safely mutateruby
mutable = frozen_str.dup
mutable << ' appended'
Sources
Official documentation ↗

Ruby Core Documentation

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