Re: advice on programming style: is multiple inheritance bad?

From: Gary Stephenson (garys_at_ihug.com.au)
Date: 02/02/04

  • Next message: Christian Seberino: "C extension=> pow(2,1) gives DIFFERENT answers in different parts of C extension!?!?! Any ideas why?" 
    Date: Mon, 2 Feb 2004 19:06:49 +1100

    Fwiw, I am both a human, and a customer of my bank. My computer is both an
    "electrical_device" and a "depreciable-asset". It is completely natural in
    the real world for objects to have multiple "is-a" relationships with
    various other objects. Why then, should we not use multiple inheritance
    when modelling such relationships?

    One major reason why multiple inheritance has such a bad name can be sheeted
    back to the brain-dead way in which C++ implements it. The other main
    reason is that inheritance itself, single or multiple, implementation or
    interface, is _way_ overrated as a code structuring technique. This is
    probably due to it not even being consistently defined - it seems to mean
    different things to different people at different times (depending on which
    language they're currently working with).

    IMHO, inheritance only works robustly in the face of change if you can
    guarantee that the potential instances of your (sub)classes form a strict
    proper subset (mathematically speaking) of the potential instances of their
    ancestor classes. afaik, which is not terribly far really
    ;-) , there is no OO language on earth that can _guarantee_ this subclass ==
    subset relationship for you - you have to arrange it yourself (somehow).

    Other folk have also suggested that in order to properly achieve the above,
    you need to follow the rule: "only abstract classes can be subclassed".
    That is, if you want to create class B as a subclass of class A, and A is
    not already an abstract class, you should create a third class A', which
    _is_ an abstract class, and have both class A and class B inherit from that.
    For a far better explanation of this and many other related issues I refer
    the interested reader to:

         http://okmij.org/ftp/Computation/Subtyping/Trouble.html

    my $A0.0148,

    gary

    "Uwe Mayer" <merkosh@hadiko.de> wrote in message
    news:bvjhkv$ss4$1@news.rz.uni-karlsruhe.de...
    > Hi,
    >
    > I got a class A2 that needs to inherit from class A1.
    > A2, as well as some other classes implement a same functionality and in
    > order to prevent code duplication I'd like to factorize that (potentially)
    > duplicate code into a class SuperA from which then A2 and all other
    classes
    > could inherrit.
    > However, this would cause A2 to have two super classes A1 and SuperA -
    which
    > is possible of course in Python.
    >
    > My question is: is that bad programming style?
    > From C++ and Java we "learn" that you shouldn't do that.
    >
    > Class A is automatically generated from other tools, so I can't solve the
    > problem by just letting A1 inherit from SuperA.
    >
    > Your opinions?
    >
    > Ciao
    > Uwe
    > --
    >

  • Next message: Christian Seberino: "C extension=> pow(2,1) gives DIFFERENT answers in different parts of C extension!?!?! Any ideas why?"

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