I agree with Goran completely on this issue.  Subclassing is
particularly in-appropriate for network protocols, since they should be
stackable.  For example, Http could be implemented by stacking the
following: 

HttpServer
SocketStream
Socket

HttpClient
SocketStream
Socket

So, an HttpClient would hold onto an a read/write stream of some sort (a
SocketStream if you're doing Http over TCP).  The stream (a SocketStream
in this case) would hold onto a Socket.  No subclassing.

If it were done carefully, you could even stack another protocol on top
of Http:

InstantMessageClient
HttpClientReadWriteStreamAdaptor
HttpClient
SocketStream
Socket

InstantMessageServer
HttpServerReadWriteStreamAdaptor
HttpClient
SocketStream
Socket

- Stephen

goran.hultgren at bluefish.se wrote:
> Hi!
> 
> I wrote:
> > >>1. I personally don't like the added
> > >>methods in Boolean like 
> > >>#yourselfUnlessFalseDoFirst: . I 
> > >>had to stare and check and think 
> > >>and almost still didn't get it! ;-) My 
> > >>advice: Get rid of those. :-)
> > > 
> > > 	Hmm, they seemed straightforward and useful to me at 
> the time. :)  
> > > I'll take another look...
>  
> > I'm glad this is mentioned. I forgot to tell you about this 
> before. I
> > removed those methods from my image a long time ago :)
> 
> :-)
>  
> > >>The Client/Server classes built for
> > >>reuse by inheritance would be 
> > >>easier to use, I think, if 
> > >>constructed for reuse by 
> > >>composition.
> > 
> > Goran, what do you mean? Can you think on any example with 
> a concrete
> > protocol (FTP, TFTP, SMTP, whatever)?
> 
> Well, I mean that the classes "Server" and "Client" are meant 
> to be used by subclassing instead of using them as 
> "instances" (they would probably need other names then).
> 
> Often OO code uses inheritance too much IMHO. Reusing code by 
> inheritance is much more brittle than reusing by composition 
> and it often requires the developer to study the source of 
> the superclass instead of relying on a good external protocol.
> 
> regards, Göran
>