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

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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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