Question

I am a little confused about object assignment and pointers in Ruby, and coded up this snippet to test my assumptions.

class Foo
    attr_accessor :one, :two
    def initialize(one, two)
        @one = one
        @two = two
    end

end

bar = Foo.new(1, 2)
beans = bar

puts bar
puts beans

beans.one = 2
puts bar
puts beans
puts beans.one
puts bar.one

I had assumed that when I assigned bar to beans, it would create a copy of the object, and modifying one would not affect the other. Alas, the output shows otherwise.

^_^[jergason:~]$ ruby test.rb 
#<Foo:0x100155c60>
#<Foo:0x100155c60>
#<Foo:0x100155c60>
#<Foo:0x100155c60>
2
2

I believe that the numbers have something to do with the address of the object, and they are the same for both beans and bar, and when I modify beans, bar gets changed as well, which is not what I had expected. It appears that I am only creating a pointer to the object, not a copy of it. What do I need to do to copy the object on assignment, instead of creating a pointer?

Tests with the Array class shows some strange behavior as well.

foo = [0, 1, 2, 3, 4, 5]
baz = foo
puts "foo is #{foo}"
puts "baz is #{baz}"
foo.pop
puts "foo is #{foo}"
puts "baz is #{baz}"

foo += ["a hill of beans is a wonderful thing"]
puts "foo is #{foo}"
puts "baz is #{baz}"

This produces the following wonky output:

foo is 012345
baz is 012345
foo is 01234
baz is 01234
foo is 01234a hill of beans is a wonderful thing
baz is 01234

This blows my mind. Calling pop on foo affects baz as well, so it isn’t a copy, but concatenating something onto foo only affects foo, and not baz. So when am I dealing with the original object, and when am I dealing with a copy? In my own classes, how can I make sure that assignment copies, and doesn’t make pointers? Help this confused guy out.

Answer 1

There are a lot of questions in this question. The main thing to know is assignment never makes a copy in ruby, but methods often return new objects instead of modifying existing objects. For immutable objects like Fixnums, you can ignore this, but for objects like arrays or Foo instances, to make a copy you must do bar.dup.

As for the array example, foo += is not concatenating onto the array stored in foo, to do that you would do foo.concat(['a']). Instead, it is making a new array and assigning foo to that. The documentation for the Array class mentions which methods mutate the array in place and which return a new array.