Ruby: the differences between dup & clone

Have you ever wondered what the differences are between #dup and #clone in Ruby?

They both create a shallow copy of an object (meaning that they don't copy the objects that might be referenced within the copied object). However, #clone does two things that #dup doesn't:

• copy the singleton class of the copied object
• maintain the frozen status of the copied object

Examples of the singleton methods not being copied.

dup:

a = Object.new
def a.foo; :foo end
p a.foo
# => :foo
b = a.dup
p b.foo
# => undefined method `foo' for #<Object:0x007f8bc395ff00> (NoMethodError)

vs clone:

a = Object.new
def a.foo; :foo end
p a.foo
# => :foo
b = a.clone
p b.foo
# => :foo

Frozen state:

a = Object.new
a.freeze
p a.frozen?
# => true
b = a.dup
p b.frozen?
# => false
c = a.clone
p c.frozen?
# => true

Looking at the Rubinius source code makes the difference extremely obvious.

Because of the extra steps, clone is a bit slower than dup but that's probably not what will make your app too slow.

Just a quick note about shallow copies (true for clone and dupe). Notice how the array referenced by the bar attribute doesn't get copied but shared between the original and the copied instances:

class Foo
  attr_accessor :bar
  def initialize
    self.bar = [1,2,3]
  end
end

a = Foo.new
b = a.clone
p a.bar
# => [1, 2, 3]
p b.bar
# => [1, 2, 3]
a.bar.clear # clearing the array #bar points to
p a.bar
# => []
p b.bar
# => []

Both objects, a and b share the same reference to the array instance created when a is initiated. There are a few ways to do a deep copy of an object, but that's a different matter.

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