[ruby-core:123131] [Ruby Bug#21538] initialize_dup not called when duping class/module

29 Aug 2025

      Issue #21538 has been updated by jeremyevans0 (Jeremy Evans).

The example given isn't a bug. `initialize_dup` is called on the new instance (in this example, the instance of `Class`), and `dup` does not copy singleton classes. If you define `Class#inititialize_dup`, it works as expected:

```ruby
class A
  def initialize_dup(_)
    puts "dup instance"
    super
  end
end
class Class
  def initialize_dup(_)
    puts "dup class"
    super
  end
end

A.new.dup
# prints "dup instance"

A.dup
# prints "dup class"
```

`dup` is different than `clone`, because `clone` copies the singleton class, and therefore will pick up singleton methods:

```ruby
class A
  def initialize_clone(_)
    puts "clone instance"
    super
  end
  def self.initialize_clone(_)
    puts "clone class"
    super
  end
end

A.new.clone
# prints "clone instance"

A.clone
# prints "clone class"
```

That being said, there are two definite bugs and another probable bug in `Class#dup`, as evidenced by this example: 

```ruby
class Class
  def initialize_dup(_)
    p ancestors
    p singleton_class.ancestors
    super
  end
end

class B
  def self.initialize_dup(_)
    puts "dup class"
    super
  end
end

class A < B
end

puts
p A.ancestors
p A.singleton_class.ancestors

puts
C = A.dup

puts
p C.ancestors
p C.singleton_class.ancestors
```

Output on Ruby 3.4 (with comments on bugs):

```
[A, B, Object, Kernel, BasicObject]
[#<Class:A>, #<Class:B>, #<Class:Object>, #<Class:BasicObject>, Class, Module, Object, Kernel, BasicObject]

# Probable Bug: During initialize_dup, ancestors are missing,
# and therefore ancestor initialize_dup singleton method not called
[#<Class:0x00000d7e572e8ca8>]
[#<Class:#<Class:0x00000d7e572e8ca8>>, Class, Module, Object, Kernel, BasicObject]

# Definite Bug 1: after dup, singleton class ancestors are missing
[C, B, Object, Kernel, BasicObject]
[#<Class:C>, Class, Module, Object, Kernel, BasicObject]
```

Output on master branch:

```
[A, B, Object, Kernel, BasicObject]
[#<Class:A>, #<Class:B>, #<Class:Object>, #<Class:BasicObject>, Class, Module, Object, Kernel, BasicObject]

# Definite Bug 2: calling singleton_class inside Class#initialize_dup
# results in TypeError
[#<Class:0x00000b7eeb88aca0>]
[#<Class:#<Class:0x00000b7eeb88aca0>>, Class, Module, Object, Kernel, BasicObject]
-:5:in 'Module#initialize_copy': already initialized class (TypeError)
        from -:5:in 'Kernel#initialize_dup'
        from -:5:in 'Class#initialize_dup'
        from -:25:in 'Kernel#dup'
        from -:25:in '<main>'
```

The reason I call the first bug probable and not definite is that it is at least defensible that ancestor setup occurs inside `dup`, but after the call to `Class#initialize_dup`. I still think it should be considered a bug and fixed. I'm guessing fixing definite bug 2 requires fixing the probable bug.

For `clone`, there is a similar issue of missing ancestors (but not singleton class ancestors) inside `initialize_clone`:

```ruby
class B
  def self.initialize_clone(_)
    p ancestors
    p singleton_class.ancestors
    puts "clone class"
    super
  end
end

class A < B
end

puts
p A.ancestors
p A.singleton_class.ancestors

puts
C = A.clone

puts
p C.ancestors
p C.singleton_class.ancestors
```

Output:

```
[A, B, Object, Kernel, BasicObject]
[#<Class:A>, #<Class:B>, #<Class:Object>, #<Class:BasicObject>, Class, Module, Object, Kernel, BasicObject]

# Probable bug: During initialize_clone, ancestors are missing
[#<Class:0x00000a047fffaab8>]
[#<Class:#<Class:0x00000a047fffaab8>>, #<Class:B>, #<Class:Object>, #<Class:BasicObject>, Class, Module, Object, Kernel, BasicObject]
clone class

[C, B, Object, Kernel, BasicObject]
[#<Class:C>, #<Class:B>, #<Class:Object>, #<Class:BasicObject>, Class, Module, Object, Kernel, BasicObject]

```

----------------------------------------
Bug #21538: initialize_dup not called when duping class/module
https://bugs.ruby-lang.org/issues/21538#change-114458

* Author: chucke (Tiago Cardoso)
* Status: Open
* ruby -v: 3.4.5
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
Not sure whether this is expected behaviour or not, but just leaving it here to start the debate on whether callbacks like `initialize_dup` are supposed to be called when a module or class is duped (the same happens with `initialize_copy` and `initialize_clone` btw):

    class A
      def initialize_dup(_)
        puts "dup instance"
        super
      end
      def self.initialize_dup(_)
        puts "dup class"
        super
      end
    end

    A.new.dup #=> "dup instance"
    A.dup #=> nothing

-- 
https://bugs.ruby-lang.org/