From: daz <dooby@...10.karoo.co.uk>
Date: 2005-09-03T12:46:26+09:00
Subject: Re: beginners YAML question


Adam Shelly wrote:
> Thanks,
> That fixed the load error, but there is still a problem
> with the to_yaml routine. The additional @size parameter
> never gets written to the YAML string.
>
> require 'yaml'
>
> class Box < Array
>   def initialize size
>     @size = size
>   end
>   def size
>     @size
>   end
> end
>
>
> box = Box.new(2)
> print "ERROR: Box, #{box.size}!=2" unless box.size == 2
> box2 = YAML::load(box.to_yaml)
> print "ERROR: Box2, #{box2.size}!=2" unless box2.size == 2
> p box.to_yaml
>
>
> ERROR: Box2, !=2
> --- !ruby/array:Box []
>
>
> Any ideas?
> -Adam

In short, @size is an attribute of Box ... but you're
using the inherited Array#to_yaml to serialise your object.

Some pointers ...

1) By defining an initialize method in Box, you /override/
   the Array initialize - so, an empty Array is allocated.
   To fix this, you need to call "super" (see 2)

2) Array already has a size attribute, so this works:

     require 'yaml'

     class Box < Array
       def initialize(*args, &block)
         super
       end
     end

     box = Box.new(3) {|x| 2*x}
     box2 = YAML::load(box.to_yaml)
     print "ERROR: box2 != box" unless box2 == box
     puts box.to_yaml

#-> --- !ruby/array:Box
#-> - 0
#-> - 2
#-> - 4

3) Note that if you /don't/ inherit from Array,
   to_yaml saves as a !ruby/object:Box along with
   all current instance variables of Box.

     require 'yaml'

     class Box
       def initialize(*args)
         @high, @wide = *args
       end
     end

     box = Box.new(4,7)
     puts box.to_yaml
     box2 = YAML::load(box.to_yaml)
     puts box2.to_yaml

#-> --- !ruby/object:Box
#-> high: 4
#-> wide: 7
#-> --- !ruby/object:Box
#-> high: 4
#-> wide: 7

# Note: I left out the "box2 != box" comparison
#  because Boxes are not comparable until you
#  define the mechanism.

4) If you want Box to behave like an Array but with
   attributes of its own, there are ways.
   Ask again (giving details) if you need help with
   that or any other aspect.


Cheers,

daz