[fpc-devel] Class extension

Matt Emson memson.lists at googlemail.com
Thu May 20 14:29:53 CEST 2010

Okay, so the whole Observer Pattern discussion this morning went way off 
track after I mentioned this idea, but a few people expressed interest 
in my proposal:

> I'd rather see a mechanism for injecting first class extensions to existing classes. That way, it really doesn't matter what pattern is implemented where. 

So, what are these "class extensions"? Some mentioned Class Helpers, ala, Delphi. This is not what I envisioned. My idea was more me "riffing" on the notions I've encountered in DotNet and Objective-C. 

In DotNet, well, C# specifically, we have the concept of "partial classes". That is to say, a class that is implemented in more than one section. In the world of DotNet, the partial classes are assembled at compile time in to a single concrete "class". The mechanism is a little clunky, and the rules prohibit some things from happening, e.g. not being able to add partial classes cross logical code block (in DotNet parlance, assembly.) What a partial class does allow the programmer to do is modularise their class structure. It is therefore possible to add/remove functionality from a class by including or excluding source files at compile time. I see this as an extremely basic version of my next item: The Objective-C Category.

In Objective-C, the Category often seems a lot like the Partial class in a superficial way. It is the same kind of idea - a breaking up of a class in to sections. Sections can be added and removed in a similar way. However the category goes further (as Objective-C always seems to) and allows functionality to be plumbed in to an existing class, without the need to alter the original source code - indeed you actually do not need the source at all. This is what I envisioned when I stated the above "class extension". It could also be seen as a mechanism to overcome the issues often encountered with an OO system lacking multiple inheritance. One of the more interesting features of Categories in Objective-C is the ability to replace methods in the base class, but using the same signature.


So, yes, this is all pie in the sky. I'm in no way making any offer or commitment making this happen. I'm also fully expecting some criticism and probably to be told it is unlikely to be implemented (or at least, only in the Objective-P dialect), but I though that I would try to spin something positive from what was discussed earlier.

To recap - this has little to do with the crippled Class Helper functionality that Borland grafted on to Delphi DotNet and then Delphi in general. This is all to do with enabling something useful and helpful.


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