DelegateClass vs. Extending

[Brian Doyle]

I was looking at DelegateClass example:

class Tempfile < DelegateClass(File)
# constant and class member data initialization...

def initialize(basename, tmpdir=Dir::tmpdir)
# build up file path/name in var tmpname...

@tmpfile = File.open(tmpname, File::RDWR|File::CREAT|File::EXCL,
0600)

# ...

super(@tmpfile)

# below this point, all methods of File are supported...
end

# ...
end

and was wondering what is the advantage of doing this and not just
extending the File class? Thanks.