From: "Bartosz Dziewoński" <matma.rex@...>
Date: 2012-04-05T23:33:08+09:00
Subject: Re: Call for comments - Structure

W dniu 5 kwietnia 2012 14:21 użytkownik Robert Klemme
<lists@ruby-forum.com> napisał:
>> Unlike Struct, it creates a "real" class, and it has real instance
>> variables
>> with predictable names.
>
> Why are Struct generated classes not real?  Or does this only refer to
> the fact that #instance_variables returns an empty list?  If so why does
> it matter?

No. Struct just does not have instance variables, which is surprising,
if you assume that "Point = Struct :x, :y" is a shorthand for "class
Point; attr_accessor :x, :y; def initialize x, y; @x, @y = x, y; end;
end".

For example this is not valid code:

Point = Struct.new :x, :y
class Point
  def distance other
    Math.sqrt((@x-other.x)**2 + (@y-other.y)**2)
  end
end

irb(main):007:0> a = Point.new 1, 2
=> #<struct Point x=1, y=2>
irb(main):008:0> b = Point.new -1, 3
=> #<struct Point x=-1, y=3>
irb(main):009:0> a.distance b
NoMethodError: undefined method `-' for nil:NilClass
        from (irb):4:in `distance'
        from (irb):9
        from F:/Ruby193/bin/irb:12:in `<main>'


-- Matma Rex