Re: Need help understanding subclassing issue

[David Chisnall]

On 20 Feb 2009, at 04:35, Pierre Bernatchez wrote:

> David Chisnall wrote:
> > On 19 Feb 2009, at 21:35, Pierre Bz wrote:
> > > I wrote a simplified (trivial) subclass of NSDictionary
> > > and it fails
> > This is expected.  As the documentation says, NSDictionary is a  
> > class cluster.  Concrete subclasses must implement these three  
> > primitive methods:
>
> The documentation I was looking at indicated none of that.
> Perhaps you could point me to the documentation you are reading.
>
> I looked up NSDictionary in the GNUstep base class reference:
> http://www.gnustep.org/resources/documentation/Developer/Base/Reference/Base.html
>
> Anyway,  thanks for the clear answer, it was exactly what I needed.

Generally, I only use the GNUstep documentation for GNUstep-specific  
features.  If you google for NSDictionary, this is the top hit:

http://developer.apple.com/DOCUMENTATION/Cocoa/Reference/Foundation/Classes/NSDictionary_Class/Reference/Reference.html

Looking at this page, it actually doesn't mention the fact that  
NSDictionary is a class cluster explicitly either.  The only hint  
given here (and in the OpenStep spec) is the section on primitive  
methods.  Generally speaking, when a class has a set of methods  
described as `primitive' you should implement them yourself in any  
subclass.

> > You should probably explain why you are trying to subclass  
> > NSDictionary though.  Most of the time, this is the wrong thing to  
> > do.
>
> I wanted to write any subclass to see if I had the right idea
> of how to do that.  I wanted to grasp how to subclass in
> a way that both observes the GNUstep initxxxx convention and
> is MAC compatible.

A lot of the Foundation classes follow this pattern.  This includes  
NSValue, NSNumber, NSString, all of the collection classes, and so  
on.  In general, subclassing is not encouraged for OpenStep programs.

> It seems I picked the wrong class for the experiment.
> I got no diagnostic message at all at first, only 'abort'.

The uncaught exception handler should have printed a diagnostic  
message before exiting.  I'm not sure why it didn't for you.

> Then with the exception handler I got the message about 'count'.
> All rather obscure until you know the solution.

This line seems quite self-explanatory:

> REASON:subclass OGBanner(instance) should override count

The OGBanner class must override the instance method `count'.

> Next time I'll leap to the cluster/abstract class issue in a flash,
> and seek better docs.

My advice with regard to documentation is to stick to using the Apple  
docs and if you find something that doesn't work on GNUstep either  
implement it and send a patch, or file a bug report.

David