Re: MI ammunition : linked lists

[Stephen Leake <Stephe.Leake@nasa.gov>]

"amado.alves" <amado.alves@netcabo.pt> writes:

> A very common programming task where multiple inheritance (MI) would
> be handy: implementation of doubly linked lists.

Hmm. I've been using doubly linked lists, and never felt a lack of MI.

> A linked list is a chain of nodes. The first node links to the
> second, the last node links to the penultimate, and each of the
> others nodes links to both the previous and the next. (Each node
> usually also carries a payload, which we can ignore here.)

Ok so far.

> Often (always?) a non OOP design is used with only one type for all
> kinds of nodes, with null values in the unused slots:
> 
>    ___________      ______               ______      ___________
>   |           |    |      |             |      |    |           |
>   | Prev=null |<-----Prev |<--- ... <-----Prev |<-----Prev      |
>   | Next---------->| Next-----> ... --->| Next----->| Next=null |
>   |___________|    |______|             |______|    |___________|
> 
> An OOP design does away with the ugly nulls (however it introduces
> one-of-a-kind objects, which also some people consider ugly):

Why is "null" ugly? It's a perfectly reasonable value.

>                ______               ______      ______
>    ______     |      |             |      |    |      |
>   |      |<-----Prev |<--- ... <-----Prev |<-----Prev |
>   | Next----->| Next-----> ... --->| Next----->|______|
>   |______|    |______|             |______|    
> 
> It is clear that the middle nodes combine the attributes of the first
> and last nodes. 

Now the "first" and "last" nodes are of a different type than the
"middle" nodes. When I insert a node, the "last" node changes type!

This is a hammer looking for a nail; I'll just use my screwdriver,
thank you.

-- 
-- Stephe