Dynamically defining the initialize method and setting the arity.

dynamic_initialize.rb

class Something
  def self.init(*init_args)

    # this works but...
    define_method(:initialize){|*args|
      # arity is -1
    }

    class_eval %Q<
      def initialize(#{*init_args})
        # this doesn't work
      end
    >
  end
end

# Here I want arity of :initialize to be 2
class Special < Something
  init(:one, :two)
end

# And here I want arity of :initialize to be 3
class Other < Something
  init(:one, :two, :three)
end

You'r forgetting class_eval takes a string...

    class_eval %Q<
      def initialize(#{init_args.join(',')})
        # this doesn't work
      end
    >

[1] pry(main)> Other.new
ArgumentError: wrong number of arguments (0 for 3)
from (eval):2:in `initialize'
[2] pry(main)> Special.new
ArgumentError: wrong number of arguments (0 for 2)
from (eval):2:in `initialize'

class Something
  def self.init(*init_args)
    define_method(:initialize) do |*args|
      # TODO: check args, raise similar exception as Ruby
      init_args.zip(args) do |name, value|
        instance_variable_set("@#{name}", value)
      end
    end
  end
end

# And here I want arity of :initialize to be 3
class Other < Something
  init(:one, :two, :three)
end

o = Other.new("one", "two", "three")
puts o.inspect
# #<Other:0x007fbbc40592b8 @one="one", @two="two", @three="three">

thanks guys! @mulder's comment work. I wasn't joining the args