From: Gennady Bystritsky <Gennady.Bystritsky@...>
Date: 2010-03-24T12:07:11+09:00
Subject: Re: what's the suggested raise Exception idiom?


On Mar 23, 2010, at 2:45 PM, Robert Klemme wrote:

> On 03/23/2010 06:22 PM, Nathan Beyer wrote:
>> What's the distinction between:
>>   raise ArgumentError, "the argument was invalid"
>> and
>>  raise ArgumentError.new("the argument was invalid")
>> 
>> How does this apply in the case of exceptions with custom initializers
>> for attributes? For example:
>>  class StatusCodeError < StandardError
>>      attr_reader :status_code
>>      def initialize(status_code)
>>          @status_code = status_code
>>      end
>>  end
>> 
>> I've been reading a lot of code, books and blogs, but haven't gotten
>> as clear of an understanding on why people choose one way over
>> another, so I'm looking for additional opinions and commentary.
> 
> I believe raise a,b,c,d... is merely syntactig sugar for raise 
> a.new(b,c,d...) with a bit of added functionality (e.g. if a is not an 
> exception class there is an error, if a is a String a StandardError is 
> raised).

Not exactly so. According to the passage from RI below, "raise" with an exception class as a first paremeter accepts up to 2 additional parameters -- the first one will be used as an argument to new when the exception object is instantiated, and the second, if any, must be an array representing callback information.

When I need an exception class with more than one argument, I use something like the following:

class DimensionError < RuntimeError
  attr_reader :width, :hight
  def initialize(*args)
    @width, @hight = *args
    super "Wrong dimension #{width}x#{hight}"
  end
end

begin
  raise DimensionError, [ 5, 3 ]
rescue DimensionError => error
  error.wigth
  error.length
end

= OR =

class DimensionError < RuntimeError
  attr_reader :width, :hight
  def initialize(options = {})
    @width = options[:width]
    @hight = options[:hight]
    super "Wrong dimension #{width}x#{hight}"
  end
end

begin
  raise DimensionError, :width => 5, :hight => 3
rescue DimensionError => error
  error.wigth
  error.length
end


----------------------------------------------------------- Kernel#raise
     raise
     raise(string)
     raise(exception [, string [, array]])
     fail
     fail(string)
     fail(exception [, string [, array]])
------------------------------------------------------------------------
     With no arguments, raises the exception in $! or raises a 
     RuntimeError if $! is nil. With a single String argument, raises a 
     RuntimeError with the string as a message. Otherwise, the first 
     parameter should be the name of an Exception class (or an object 
     that returns an Exception object when sent an exception message). 
     The optional second parameter sets the message associated with the 
     exception, and the third parameter is an array of callback 
     information. Exceptions are caught by the rescue clause of 
     begin...end blocks.

        raise "Failed to create socket"
        raise ArgumentError, "No parameters", caller
> 
> To answer your question, normally there is no reason to use a variant 
> with "new" - rather save yourself a bit of typing and use the shorter 
> form.  My 0.02EUR.
> 
> Kind regards
> 
> 	robert
> 
> -- 
> remember.guy do |as, often| as.you_can - without end
> http://blog.rubybestpractices.com/
>