From: Mario Antonetti <mjtanton@...>
Date: 2010-04-07T00:23:05+09:00
Subject: Re: Custom Exceptions

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On Tue, Apr 6, 2010 at 9:44 AM, Leslie Viljoen <leslieviljoen@gmail.com>wrote:

> Hi everyone
>
> I want to make a custom exception like so:
>
> class BillRowError < StandardError
>    def initialize(field, index)
>        @field = field
>        @index = index
>    end
> end
>
> I'll call this like so:
> raise(BillRowError.new(:roamingcalls, @index), "Roaming Calls field
> missing") if n.length == 0
>
> But now I'd like to be able to modify the string that Ruby prints when the
> exception is not rescue'd. I thought I could add this method to the
> BillRowError class:
>
>    def message
>        @message + " field: #{@field}, row: #{@index}"
>    end
>
> That almost works but I get a "instance variable @message not initialized"
> warning, which means Ruby is not setting @message in my object like I
> expected. Making my own message= method doesn't help.
>
> Can an exception object access and modify the message that gets passed in
> the "raise"?
>

You could try rewriting the to_s method and take advantage of the
inheritance:

def to_s
  super + " field: #{@field}, row: #{@index}"
end

That worked for me.  I'm not sure if this is the recommended solution.

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