Aspect-Oriented Programming Considered Harmful

kupci writes "The 'x considered harmful' cry is a little overused, but there is a Forrester report that discusses some of the pro's and con's of Aspect-oriented Programming, and includes some interesting links. It is mainly based on papers from the University of Passau. It's worth it just for Clark's 'COME FROM' article." From the article: "Aspect-oriented programming (AOP) is intended to address common problems that object-oriented programming (OOP) doesn't address well, plus some problems that OOP itself created. However, AOP is a risky solution: It is a very generic mechanism for solving some very specific concerns and has been likened to a kind of "GOTO" statement for OOP. Like GOTO, it can cause more harm than good."

Comments

For those unfamiliar with AOP

[ravenspear]

Wikipedia linkage

In computing, the programming paradigm of aspect-oriented programming (AOP) centers on constructs called aspects, which treat concerns of objects, classes, or methods. The aim of AOP is to separate program code related to the main purposes of the application (its core concerns) from code related to secondary purposes (cross-cutting concerns).

For example, a telecommunications application might have a core concern of routing calls, while code for timing and billing those calls would crosscut the whole object hierarchy. AOP aims to separate the billing concerns from the core concern. It moves code not related to solving the domain problem from the business logic into a separate module. The application code no longer contains pieces of crosscutting concerns scattered across modules; instead, programmers maintain crosscutting concerns in aspects; this makes it easier to maintain both core and crosscutting concerns.

Any program has principled points (join points) where programmers can identify and modify the program semantics. In AOP, programmers specify join points using a language feature called a pointcut, and specify the behavior to join those points by using advice such as methods or functions. Some variants of AOP allow programmers to extend the types in the system. These features enable aspects to implement behavior for concerns that crosscut the core concern of the application.

Re:I like GOTO!

[top_down]

You have to catch [execeptions] at every point to free stuff, and the nested structures become even uglier than gotos.

The freeing of resources, all resources not just memory, is automatic in C++ once you learn to use RAII.

Look it up. It is one of major improvements of standard C++ over C/C++.

Objective-C has these, and no one bats an eyelash.

[israfil_kamana]

A very well respected (if a bit obscure) OO language is Objective-C. It has a few features that are analogous to aspects.

First it has categories, which allow you to add new methods, or replace the implementation of a method in a given class. You add a category to that class that includes, say, a replacement description method, and the new implementation replaces the old. Or you can add a path utility methods to the String class, as another example.

It also has "poseAs", which allows you to insert an object into a hierarchy. Say you have Class B which extends Class A. But you want custom behaviour to be accessible to all As children (including, for example, B). You create Class C, which extends A, and make it poseAs Class A. Now the runtime hooks all clesses that inherit A off of C, which replaces A. A still exists, but is in the background, providing normal inherited functionality to C.

These abilities were one of the things that made Objective C a very powerful OO language. In particular, it allows a particular kind of reuse, by virtue of allowing one to patch someone else's library without having source. So if you want to add the aforementioned path utilities to NSString, you just added a Category in your own code, and your code would run with the extended functionality in NSString (no one elses would, unless they used your category too). This means that often you can make the most out of someone else's code without a complete rewrite.

It isn't all powerful. There are access and visibility restrictions. It is also used to partition code for ease of organization and better modularization. Here's the point:

Because it is a well known feature of the language, you think in terms of categories, and you know to look for categories. This can be aided by tools, but you most definitely need to pay attention to them. But because of this, they are only "dangerous" if you aren't paying attention. And guess what! That's true of programming, period.