How do you create a custom subclass of IO in Ruby?

Question

How do I make my own class that can be substituted for IO in order to e.g. redirect/capture input/output for some code that accepts an IO-like parameter? IO itself seems to be coupled to OS file descriptors and the only class I know of that mimics it without subclassing it is StringIO, which seems to just reimplement the entire interface.

I would think that the language would provide a straightforward way to do this but I can't find any information on the subject. Is there a mixin that implements the interface on top of a few primitives, as Enumerable does?

Answer 1

I suspect I'm missing something, but supposing that's not the case...

Why would subclassing not be an option? Could you not just override the parts of IO that need to behave differently?

class MyIO < IO
  # your modified methods go here. e.g.
  def some_io_method(args)
    do_some_preprocessing
    super(args)
    do_some_post_processing
  end
end

If you can't substitute your home-brew subclass, what about modifying IO itself ("monkey-patching") something like:

class IO
  alias_method :original_some_io_method, :some_io_method
  def some_io_method(args)
    do_some_preprocessing
    original_some_io_method(args)
    do_some_post_processing
  end
end

Answer 2

I have successfully used the following hack to make StringIO a "real" IO:

    # make mechanize believe our address xml actually is an IO so it will be uploaded as a file:
    address_io = StringIO.new(address_xml)
    class << address_io
      alias is_a_old is_a?
      def is_a?(stuff)
        stuff == IO || is_a_old(stuff)
      end
      def path
        'address.xml'
      end
    end