inheriting from StandardError

I often see custom Exception classes inheriting from StandardError. Errors which you can generally deal with are subclassed from StandardError:

module AbstractController   class Error < StandardError #:nodoc:   end

  class ActionNotFound < StandardError #:nodoc:   end

unless action_name = method_for_action(action_name)   raise ActionNotFound, "The action '#{action}' could not be found for #{}" end

But why even bother to create the subclasses (which as shown above, have no definition of themselves), when substituting ActionNotFound with StandardError during the raise call will have the same effect.

I would understand if there was some level of customization involved that distinguishes it from the super class:

class DoubleRenderError < Error     DEFAULT_MESSAGE = "Render and/or redirect were called multiple times in this action. Please note that you may only call render OR redirect, and at most once per action. Also note that neither redirect nor render terminate execution of the action, so if you want to exit an action after redirecting, you need to do something like \"redirect_to(...) and return\"."

    def initialize(message = nil)       super(message || DEFAULT_MESSAGE)     end   end

But in the exampels I provided in the initial post that was not the case. ActionNotFound, for example, had no implementation of its own. It seems to be a waste of memory allocation.

Because it doesn’t have the same effect. Raising specific subclasses allows you to write targeted rescue clauses rather than rescuing indiscriminately.


So we must build an entire class just for sake of more specific rescue statements:

class MyError < StandardError end

try_counter = 0 begin try_counter += 1 puts 'Here 1' raise "Text" unless try_counter > 5 puts 'Here 2' rescue MyError puts 'Here 3 - MyError encountered' retry rescue StandardError puts "Here 4 - Other error encountered (#{$!.inspect})" + caller.inspect raise else puts 'Here 5 - No errors' ensure puts 'Here 6 - Always done' end