What are the advantages to using StringIO in Ruby as opposed to String?

Basically, it makes a string look like an IO object, hence the name StringIO.

The StringIO class has read and write methods, so it can be passed to parts of your code that were designed to read and write from files or sockets. It’s nice if you have a string and you want it to look like a file for the purposes of testing your file code.

def foo_writer(file)
  file.write "foo"
end

def test_foo_writer
  s = StringIO.new
  foo_writer(s)
  raise 'fail' unless s.string == 'foo'
end