The Book of Ruby

Saving Singletons

Let’s take a look at a concrete example of a problem mentioned earlier, namely, the inability to use marshaling to save and load a singleton. In singleton_m.rb I have created an instance of Object, ob, and then extended it in the form of a singleton class that is given the additional method, xxx:

singleton_m.rb

ob = Object.new

class << ob
    def xxx( aStr )
        @x = aStr
    end
end

The problem arises when I try to save this data to disk using Marshal.dump. Ruby displays an error message: “singleton can’t be dumped (TypeError).”

YAML and Singletons

Before considering how you might deal with this, let’s briefly take a look at how YAML would cope in this situation. The program singleton_y.rb tries to save the singleton that I created a moment ...