What's the difference between Ruby's dup and clone methods?

Subclasses may override these methods to provide different semantics. In Object itself, there are two key differences.

First, clone copies the singleton class, while dup does not.

o = Object.new
def o.foo
  42
end

o.dup.foo   # raises NoMethodError
o.clone.foo # returns 42

Second, clone preserves the frozen state, while dup does not.

class Foo
  attr_accessor :bar
end
o = Foo.new
o.freeze

o.dup.bar = 10   # succeeds
o.clone.bar = 10 # raises RuntimeError

The Rubinius implementation for these methods is often my source for answers to these questions, since it is quite clear, and a fairly compliant Ruby implementation.

When dealing with ActiveRecord there's a significant difference too:

dup creates a new object without its id being set, so you can save a new object to the database by hitting .save

category2 = category.dup
#=> #<Category id: nil, name: "Favorites"> 

clone creates a new object with the same id, so all the changes made to that new object will overwrite the original record if hitting .save

category2 = category.clone
#=> #<Category id: 1, name: "Favorites">

One difference is with frozen objects. The clone of a frozen object is also frozen (whereas a dup of a frozen object isn't).

class Test
  attr_accessor :x
end
x = Test.new
x.x = 7
x.freeze
y = x.dup
z = x.clone
y.x = 5 => 5
z.x = 5 => TypeError: can't modify frozen object

Another difference is with singleton methods. Same story here, dup doesn't copy those, but clone does.

def x.cool_method
  puts "Goodbye Space!"
end
y = x.dup
z = x.clone
y.cool_method => NoMethodError: undefined method `cool_method'
z.cool_method => Goodbye Space!