Weird behaviour copying objects

Question

I'm trying to deep copy an object which holds a 3D array. But it isn't working as expected.

class A
    def initialize
        @a = [[[1]], [[2]]]
    end

    def modify3D
        @a[0][0] = @a[1][0]
    end

    def modify2D
        @a[0] = @a[1]
    end

    def dup
        copy = super
        copy.make_independent!
        copy
    end

    def make_independent!
        @a = @a.dup
    end

    def show
        print "\n#{@a}"
    end
end

a = A.new
b = a.dup
#a.modify2D
b.show
a.modify3D
b.show

In this case b is changed by the modify3D call on a.

[[[1]], [[2]]]
[[[2]], [[2]]]

If I uncomment modify2D line everything works fine.

[[[1]], [[2]]]
[[[1]], [[2]]]

Could someone pls explain what is happening here?

Answer 1

Your copy isn't deep enough. Array#dup only duplicates the array itself, not the array elements. You end up with two different arrays that still share the same elements.

In this answer I showed how to use marshaling to do a deep copy. This method does all the work:

def deep_copy(o)
  Marshal.load(Marshal.dump(o))
end

deep_copy works for any object that can be marshaled. Most built-in data types (Array, Hash, String, &c.) can be marshaled. Any object which instance variables can be marshaled can itself be marshaled.

Having defined deep_copy, then replace this line:

b = a.dup

with this:

b = deep_copy(a)

and it ought to work as you expect.