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From: jean@paradim.UUCP (Jean Pierre LeJacq)
Newsgroups: comp.lang.c++
Subject: Re: Design Problem
Summary: Using polymorphism
Message-ID: <192@paradim.UUCP>
Date: 11 Dec 89 01:09:37 GMT
References: <22137@brunix.UUCP>
Organization: Rush Systems Inc., Milford, NH
Lines: 24

In article <22137@brunix.UUCP>, sdm@cs.brown.edu (Scott Meyers) writes:
> I find myself with a collection of objects of a given type, where some of
> them define a particular function and some of them don't, and I want to
> invoke the function on only those objects for which it is defined. ....
> 
> It seems I have three possible approaches:
>     1.  Define evaluate() as a virtual function in ARC, and redefine it
>         appropriately in EXECUTABLE_ARC.  In ARC, make it a noop, so when
>         called on NONEXECUTABLE_ARCs, it won't do anything.
>     2.  Add an enumerated type to ARC that enumerates each of the subtypes,
>         and add a virtual function arc_type() that is redefined in each
>         subclass to return the appropriate value. ...
>     3.  Separate my ARC class into two distinct classes, so I never have
>         this problem.

Another approach is to insure that only instances of EXECUTABLE_ARC
are inserted into a collection. You are then assured that all members
of the collection, including public subclasses, have evaluate() defined.

This will require redesigning the solution to your problem at a higher
level. For example, two collections may be required to manage the
two fundamental ARC classes. The goal is to use polymorphism to your
advantage instead of reverting to C "switch" approaches or to de-tune
your class definitions.