From: John Carter <john.carter@...>
Date: 2002-01-31T11:38:41+09:00
Subject: Policy Classes - Challenge

Alexandrescu's book Modern C++ Design
  http://cseng.aw.com/book/0,3828,0201704315,00.html
describes (See the "Sample Chapter" link) the concept of Policy Classes.

I see this, like the laser was, a "Solution looking for a problem".

Basically it is the idea that the client of a library should...
  * Not be forced to inherit from library objects.
  * Should be able to plug in any suitable class provided by the
    client or the library as the base class of a library class.
  * Should be able to plug in several orthoganal policy classes as
    multiply inherited base classes.

He does all this magic by doing arcane things with C++ templates. (Michael
Feathers mostly wondrously called the book "Porn for Programmers".)

I believe that Ruby mixins whould be able to be able to do all the above
in a far far cleaner manner. Indeed item one is the obvious thing about
mixins.

My only problem is to think of a small, neat and compelling demonstration
of these concepts. His examples (with exception of threading policy) all
relate to C++ arcana that simply are "not an issue" in Ruby.

Question. Can you think of a neat and compelling use for this concept, I'm
prepare to write the code as a proof of concept.

John Carter                             Phone : (64)(3) 358 6639
Tait Electronics                        Fax   : (64)(3) 359 4632
PO Box 1645 Christchurch                Email : john.carter@tait.co.nz
New Zealand

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