Ruby Marshal

Marshal.dump packs an object into a string, and Marshal.load restores it. The restored copy carries the same values but is a separate object.

A custom object round-trip

The Book class stores a title behind a reader. The copy restored from the packed string answers the same title.

class Book
  attr_reader :title
  def initialize(title)
    @title = title
  end
end
book = Book.new("Emma")
copy = Marshal.load(Marshal.dump(book))
puts copy.title

A mixed array round-trip

The array mixes a number, a string, a symbol and a nested array. The restored array prints the same values and compares as equal.

values = [1, "two", :three, [4, 5]]
restored = Marshal.load(Marshal.dump(values))
p restored
puts values == restored

The copy is a different object

The copy and the original hold the same title, so the title comparison is true. equal? checks identity, so it gives false.

class Book
  attr_reader :title
  def initialize(title)
    @title = title
  end
end
book = Book.new("Emma")
copy = Marshal.load(Marshal.dump(book))
puts copy.title == book.title
puts copy.equal?(book)

Nested values are copied too

The Basket object holds an array of items. Adding an item to the copy leaves the array inside the original basket untouched.

class Basket
  attr_reader :items
  def initialize(items)
    @items = items
  end
end
basket = Basket.new(["pen", "book"])
copy = Marshal.load(Marshal.dump(basket))
copy.items << "ruler"
p copy.items
p basket.items

Example

class Label
  attr_reader :name
  def initialize(name)
    @name = name
  end
end
labels = [Label.new("first"), Label.new("second")]
restored = Marshal.load(Marshal.dump(labels))
puts restored[1].name
puts restored[0].equal?(labels[0])

Key points

Marshal.dump turns an object into a packed string.
Marshal.load turns that string back into an object.
A restored custom object answers its readers as before.
The restored object is a separate object from the original.
equal? compares identity, so a restored copy gives false.
Nested values inside a container are packed and restored too.

In this example, an array holds two Label objects, and the whole array is packed and restored in one step. The restored second label still answers its name reader. The equal? check shows the restored label is a separate object from the original.

Try it Yourself

Edit the code, then press Run.

Running is simulated

class Label
  attr_reader :name
  def initialize(name)
    @name = name
  end
end
labels = [Label.new("first"), Label.new("second")]
restored = Marshal.load(Marshal.dump(labels))
puts restored[1].name
puts restored[0].equal?(labels[0])

Exercise

An object is packed with Marshal.dump and restored with Marshal.load. What does copy.equal?(original) give?

false true nil The packed string