Sub-classing Fixnum in ruby

Question

So I understand you aren't supposed to to directly subclass Fixnum, Float or Integer, as they don't have a #new method. Using DelegateClass seems to work though, but is it the best way? Anyone know what the reason behind these classes not having #new is?

I need a class which behaves like a Fixnum, but has some extra methods, and I'd like to be able to refer to its value through self from within the class, for example:

class Foo < Fixnum
  def initialize value
    super value
  end

  def increment
    self + 1
  end
end

Foo.new(5).increment + 4 # => 10

Answer 1

You can pretty easily set up a quick forwarding implementation yourself:

class MyNum
  def initialize(number)
    @number = number
  end

  def method_missing(name, *args, &blk)
    ret = @number.send(name, *args, &blk)
    ret.is_a?(Numeric) ? MyNum.new(ret) : ret
  end
end

Then you can add whatever methods you want on MyNum, but you'll need to operate on @number in those methods, rather than being able to call super directly.

Answer 2

IIRC, the main implementation of Ruby stores Fixnums as immediate values, using some of the low bits of the word to tag it as a Fixnum instead of a pointer to an object on the heap. That's why, on a 32-bit machine, Fixnums are only 29-bits (or whatever it is) instead of a full word.

So because of that, you can't add methods to a single "instance" of Fixnum, and you can't subclass it.

If you're dead-set on having a "Fixnum-like" class, you'll probably have to make a class that has a Fixnum instance variable, and forward method calls appropriately.