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

ruby clone dup
cali-1337500 picture cali-1337500 · Apr 17, 2012 · Viewed 86.8k times · Source

The Ruby docs for dup say:

In general, clone and dup may have different semantics in descendent classes. While clone is used to duplicate an object, including its internal state, dup typically uses the class of the descendent object to create the new instance.

But when I do some test I found they are actually the same:

class Test
   attr_accessor :x
end

x = Test.new
x.x = 7
y = x.dup
z = x.clone
y.x => 7
z.x => 7

So what are the differences between the two methods?

Answer

Jeremy Roman picture Jeremy Roman · Apr 17, 2012

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.

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