From: David Alan Black <dblack@...>
Date: 2001-06-29T07:31:14+09:00
Subject: [ruby-talk:17037] keeping an Exception object alive

Hello --

This started out as an attempt to answer ruby-talk:17010, but it has
taken on a life of its own :-)

I wanted to use a particular exception object as the first argument to
raise, in order to keep that particular object alive:

  def etest
    [].implode             # trigger NameError
  rescue NameError => ne
    p ne.id
    raise ne
  end

  begin
    etest
  rescue NameError => ne
    p ne.id
  end

  => 537630972
     537630972


Then I wanted to add a message, so I made this change:

   - raise ne
   + raise ne, "some message"

This time I got different objects:

  => 537630942
     537630872

OK... so a new NameError object is being created, based on the class
of ne.  A workaround is possible, if one really wants to keep the same
object alive:

   rescue NameError => ne
     ne = ne.exception("some message")
     p ne.id
     raise ne

but there's a bit of a POLS issue here, at least for me.  (It's all
working as documented -- it's the way it works that provides the S :-)

What I'm left wondering is why there's no Exception#message= method,
which could be used directly and/or perhaps in the background of
Exception#exception.  (So that 'raise ne, "message"' would raise ne
itself, first changing its message, rather than creating a new
object.)


David

-- 
David Alan Black
home: dblack@candle.superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav