Stricter OOP using AOP!

An application I wrote recently was exhibiting a strange bug. When a user added contacts to the contacts database, details from previously entered contacts were being inserted in fields that were left blank for the new contact. The client had entered over a hundred contacts before they realised that this was happening and were somewhat worried. After some investigation, I tracked down the cause of the problem: an accidentally unscoped variable in a CFC method.

In Object Oriented languages, an object’s properties are explicitly declared in its definition; they are integral to the make-up of the object. To add or remove an object’s properties during its life could be seen as mutating the object and an undesirable behaviour in a strict OO design (I do not believe it is possible to do this in C++ but I may be mistaken).

Due to the way we emulate OO in ColdFusion, such mutation is exactly what happened when I clumsily used an unscoped variable in my CFC; the variable became part of my object’s ‘integral make-up’ resulting in the sneaky bug.

This made me wonder: how could I restrict my objects from ever modifiying their definitions by deleting or creating new variables in the variables or this scope during their existance?

The first, and so far only, useable answer I have arrived at is through using Aspect Oriented Programming (AOP). My implementation uses the ColdSpring framework although it could be done without it. For the AOP savvy, I simply created an around advice object that checks the target object’s properties before and after any method execution - if there is an inconsistency, an appropriate error is thrown. The pseudo code looks like this:

BEGIN SET variablesBefore = Get object's variables EXECUTE object's method SET variablesAfter = Get object's variables IF variablesBefore <> variablesAfter THEN throw error END

If anyone is interested in the Source code I can post it, just a PITA to put code up using blogger! Better solutions on a postcard.

Dominic