From: Chris Hulan <chris.hulan@...>
Date: 2010-04-07T00:25:07+09:00
Subject: Re: Custom Exceptions

On Apr 6, 10:44 am, Leslie Viljoen <leslievilj...@gmail.com> wrote:
> [Note:  parts of this message were removed to make it a legal post.]
>
> 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"?

The default Exception:initialize is defined to take 1 parameter, a
string containing the error message
You define different initialize parameters so @message is not getting
set as expected

Maybe something like:
class BillRowError < StandardError
    def initialize(msg, field, index)
        super(msg)
        @field = field
        @index = index
    end

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

end
...
raise(BillRowError.new("Roaming Calls field missing",:roamingcalls,
@index), ) if n.length == 0

cheers