Which Ruby classes support .clone?

Question

Ruby defines #clone in Object. To my suprise, some classes raise Exceptions when calling it. I found NilClass, TrueCla

Answer 1

I still don't know how to test for clonability properly but here's a very clunky, evil way to test for clonablity using error trapping:

def clonable?(value)
  begin
    clone = value.clone
    true
  rescue
    false
  end
end

And here's how you can clone even the unclonable. At least for the very few classes I've tired it with.

def super_mega_clone(value)
  eval(value.inspect)
end

Here's some sample testing:

b = :b
puts "clonable? #{clonable? b}"

b = proc { b == "b" }
puts "clonable? #{clonable? b}"

b = [:a, :b, :c]
c = super_mega_clone(b)

puts "c: #{c.object_id}"
puts "b: #{b.object_id}"
puts "b == c => #{b == c}"
b.each_with_index do |value, index|
  puts "[#{index}] b: #{b[index].object_id} c: #{c[index].object_id}"
end
b[0] = :z

puts "b == c => #{b == c}"
b.each_with_index do |value, index|
  puts "[#{index}] b: #{b[index].object_id} c: #{c[index].object_id}"
end

b = :a
c = super_mega_clone(b)
puts "b: #{b.object_id} c: #{c.object_id}"

> clonable? false
> clonable? true
> c: 2153757040
> b: 2153757480
> b == c => true
> [0] b: 255528 c: 255528
> [1] b: 255688 c: 255688
> [2] b: 374568 c: 374568
> b == c => false
> [0] b: 1023528 c: 255528
> [1] b: 255688 c: 255688
> [2] b: 374568 c: 374568
> b: 255528 c: 255528