From: Brian Candler <b.candler@...>
Date: 2010-04-07T00:24:39+09:00
Subject: Re: Custom Exceptions

Judging by the C source, Exception uses hidden(*) instance variables 
"mesg" and "bt" for message and backtrace respectively. However, the 
accessor #message internally calls #to_s, which you can override.

class BillRowError < StandardError
  def initialize(field, index)
    @field = field
    @index = index
  end

  alias :orig_to_s :to_s
  def to_s
    "#{orig_to_s} field: #{@field}, row: #{@index}"
  end
end

begin
  raise BillRowError.new(5, 7), "bar"
rescue BillRowError => e
  p e
  p e.instance_variables
  p e.message
  p e.backtrace
end

Produces:

#<BillRowError: bar field: 5, row: 7>
["@index", "@field"]
"bar field: 5, row: 7"
["ert.rb:14"]

Admittedly this behaviour doesn't seem to be well documented, so may be 
fragile. Use at your own risk.

HTH,

Brian.

(*) It uses instance variables whose names don't begin with '@' and so 
cannot be accessed by normal code
-- 
Posted via http://www.ruby-forum.com/.