CODEWITHSUNDEEP
rubyinheritance
cloudhead
cloudhead

Reputation: 15343

Sub-classing Fixnum in ruby

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

Upvotes: 7

Views: 4337

Answers (3)

Yehuda Katz
Yehuda Katz

Reputation: 28703

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.

Upvotes: 17

Jason C
Jason C

Reputation: 22067

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.

Upvotes: 4

Turnor
Turnor

Reputation: 1856

Could you not extend FixNum itself? Like...

class Fixnum
  def even?
    self % 2 == 0
  end
end

42.even?

Upvotes: 2

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