[fpc-devel] Class extension

[Jonas Maebe]

On 20 May 2010, at 14:29, Matt Emson wrote:

> 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.

Having implemented support for Objective-C/Pascal categories in the  
compiler, I don't think that this concept can be translated to the  
(Delphi-compatible) Object Pascal model/implementation. It only works  
in Objective-C/Pascal because there all dispatching is based on the  
name of the method/selector (instead of based on VMT offsets or static  
binding).

It may be doable with a separate class hierarchy (like  
TInterfacedObject for COM) and compiler/RTL magic (that would probably  
duplicate large parts of the Objective-C run time's functionality to  
get everything running at a reasonable speed), but in that case it  
might be more worthwhile to add support for GNUStep and extend the  
Objective-Pascal support to all platforms (since it wouldn't work with  
the regular Object Pascal classes in the RTL anyway). Objective-Pascal  
and Object Pascal can be mixed in the same file, so you can use  
delegation and other ways to bridge the two kinds of classes.


Jonas