🚀 go-pugleaf

branzcgp@branz.org.nz (Christopher Price) wrote:
>On Sun, 08 Oct 1995 03:00:48 -0600, Nelson Yu
><nyu@gpu.srv.ualberta.ca> wrote:
>>
>[snap snap]
>
>>Albeit you can't compare OWL with AWT, which has fairly 
>>complicated hierarchy that uses multiple inheritance extensively, 
>>'cause they are suited for different purposes.
>
>Many of the methodology gurus have stated (sorry I'm not chasing the 
>reference here) that single inheritence leads to poor design.
>
>This axiom (I've taken it to heart) is one foundation for good OO
>technology of the future  for complex projects.
<<munch>>

OK I'll bite ;)  Java supports multiple inheritance of interface, but only 
single inheritance of implementation (class).  At the design level, you can 
treat this a multiple inheritance (and _implement_ multiply inherited 
interfaces using delegation).

In a statically-typed OO language, you have to have either:

* parameterized types (templates)

-- or --

* interfaces (or protocols)

..to make up for losing runtime method dispatch.  

Templates are cool because they allow you to use _existing_ classes in 
interesting ways without requiring changes to those classes (changes like 
adding a mixin base-class, or specifying an additional interface to implement 
or protocol to adopt).

Interfaces are cool because they are not templates (grin).  Seriously though, 
current C++ language systems (and build systems) are notoriously bad at 
"doing" templates.  This becomes a real problem when we start to use STL and 
build our systems in an STL style (templates everywhere).

So, I will agree that in the _absence_ of templates and interfaces, 
single-inheritance leads to poor design (because there are certain things you 
simply cannot do, and because reuse often requires changes to existing 
classes).

With templates, however the need for multiple inheritance goes away.  I don't 
have to use MI to use STL for instance.  I don't have to use MI to 
_implement_ STL.  With interfaces (protocols) MI (of interface -- not 
implmentation) is possible and useful.  The one problem being that it is 
sometimes necessary to subclass an existing class just to add an interface 
(or worse yet, we sometimes actually add the new interface to the existing 
class).  Whew!

-- 
Bill Burcham                  billb@sunquest.sunquest.com
                              520.733.6361 (Work)
Sunquest Information Systems  520.733.6602 (Fax)