From: Leslie Viljoen <leslieviljoen@...>
Date: 2010-04-07T06:58:10+09:00
Subject: Re: Custom Exceptions

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On Tue, Apr 6, 2010 at 5:24 PM, Brian Candler <b.candler@pobox.com> wrote:

> 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.
>
>
Wow thanks for this, I didn't think of aliasing the old to_s. I find it
strange that exceptions don't just use a plain old @message instance
variable but I suppose there's some good reason for it.


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