Sunday, April 26, 2009

Forget Me Not! Object Oriented Design in Custom Exception

When designing custom exceptions, you may forget about old school OO fundamentals. As a reminder, lets take a look into the following custom exception classes.


These exceptions are really useful in my application. But the bad thing is, they all derive from StandardError class, whereas there should be a base class, may be StoryError, which is more meaningful and useful. So, we can have the following-

class StoryError < StandardError
StorySaveError < StoryError
StoryDescopeNotAllowedError < StoryError
StoryCompleteError < StoryError
StoryNotFoundError < StoryError
StoryDeleteError < StoryError
StoryDeleteNotAllowedError < StoryError

With the addition of a base class for all Story related exceptions, I can write cleaner/better code.

  • When I am not interested about the specific error, I can just use rescue StoryError to catch 'em all
  • I can place a common error handling routine for all Story related exceptions
  • I can add new exception types without altering the codes where the specific type of StoryError is insignificant

From my experience, I found that most people are not cautious about OO when desiging custom exceptions. (because they are exceptions!). Nonetheless, if you follow the OO guides, it will pay off for sure.