Assigning random data for class vs metaclasses

class.rb

class Animal
  def initialize
    @heads = rand(10)
    @legs = rand(10)
  end
end

# irb(main):001:0> Animal.new
# => #<Animal:0x88cf1b0 @heads=3, @legs=4>
# irb(main):002:0> Animal.new
# => #<Animal:0x8b8b0fc @heads=5, @legs=9>

metaclass.rb

class Metaid
  def self.metaclass; class << self; self; end; end

  def self.traits( *arr )
    return @traits if arr.empty?

    attr_accessor( *arr )

    arr.each do |a|
      metaclass.instance_eval do
        define_method( a ) do |val|
          @traits ||= {}
          @traits[a] = val
        end
      end
    end

    class_eval do
      define_method( :initialize ) do
        self.class.traits.each do |k,v|
          instance_variable_set("@#{k}", v)
        end
      end
    end
  end
end

class Animal < Metaid
  heads rand(10)
  legs rand(10)
end

# irb(main):001:0> Animal.new
# => #<Human:0x8138370 @heads=8, @legs=2>
# irb(main):002:0> Animal.new
# => #<Human:0x83ee790 @heads=8, @legs=2>

You need to be aware of when a method is called.

Let's look at this line:

class Animal < Metaid
  heads rand(10)   # THIS LINE
end

This line does this: Animal.heads(rand(10)). It's just a regular method call. In a method call, all arguments are evaluated first. So it will first call rand(10) (and let's say it returns 8) and then it will invoke the Animal.heads method with 8 as the first argument.

The method Animal.heads looks like this:

define_method ( a ) do |val| # a = :heads is this method
  @traits ||= {}
  @traits[a] = val
end

Let's look at the case for Animal.heads(8):

# This is the values that are set
a = :heads
val = 8

# This is the method body
@traits ||= {}
@traits[a] = val

# The result is thus:
@traits ||= {}
@traits[:heads] = 8

Notice how the information about rand(10) is completely "gone".

You can do this same exercise for the class which does rand(10) in the initializer to see why it behaves differently.