From: Brian Candler <b.candler@...>
Date: 2011-04-21T18:41:29+09:00
Subject: Re: Using the spaceship operator

RichardOnRails wrote in post #994206:
> Class MyTest < Array
>     def <=>(other)
>   self[1] <=> other[1]
>     end
>
>     @sorted_array = $date
>  end
>
> How can I make this work?

Your way *can* be made to work, but it's ugly.

class MyElement < Array
  def <=>(other)
    self[1] <=> other[1]
  end
end

a = [ MyElement.new([:A,12]),
      MyElement.new([:B,30]),
      MyElement.new([:C,4]),
    ]
a.sort!
p a

The point is that the <=> operator is applied to pairs of elements in 
the array, not to the container object itself.

As a general rule I'd say: don't subclass standard Ruby classes. Use 
delegation instead. So if you want a container object with Array-like 
properties but special behaviour, then make it have an Array, rather 
than be an Array.

class MyContainer
  def initialize(a = [])
    @a = a
  end
  def sort!
    @a = @a.sort_by { |elem| elem[1] }
  end
  ... delegate other methods as required,
  ... or use SimpleDelegator
end

In the longer term you'll find this a much more flexible way of 
composing objects and behaviour. The only reason programming classes 
tell you to subclass is because of the inflexible typing systems in 
certain other languages.

Ruby has "duck typing": there is no need for your class to inherit from 
Array, it just has to implement the behaviour of Array that you're 
interested in.

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