From: Brian Candler <b.candler@...>
Date: 2010-04-07T00:56:17+09:00
Subject: Re: Custom Exceptions

Chris Hulan wrote:
> The default Exception:initialize is defined to take 1 parameter, a
> string containing the error message

But 'raise' also lets you pass an error message, in addition to the 
exception instance.

Note that even the default constructor doesn't create an instance 
variable called @message.

class Foo < StandardError
end

begin
  raise Foo, "bar"
rescue Foo => e
  p e
  p e.instance_variables
  p e.instance_variable_get(:@message)
  p e.message
  p e.backtrace
end

#<Foo: bar>
[]              # << No instance variables!
nil             # << No @message!
"bar"
["ert.rb:5"]
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