NetNews Usenet Archive 1993 #3/NN_1993_3.iso/spool/comp/lang/cplus/20013

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Xref: sparky comp.lang.c++:20013 comp.object:5087
Path: sparky!uunet!charon.amdahl.com!netcomsv!netcomsv!ulogic!hartman
From: [e-mail] (Richard M. Hartman)
Newsgroups: comp.lang.c++,comp.object
Subject: Re: HELP : Array of objects of diff classes ??
Message-ID: <[e-mail]>
Date: 27 Jan 93 19:06:26 GMT
References: <[e-mail]>> <[e-mail]> <[e-mail]>
Followup-To: comp.lang.c++
Organization: negligable
Lines: 61

In article <[e-mail]> [e-mail] (Mike Campbell ) writes:
|[e-mail] (Terry Campbell) writes:
|
|
|>In article <[e-mail]> [e-mail] (John MAX Skaller) writes:
|
|>>Dear Everybody,
|>>
|>>
|>>    I have a problem related to programming in an object oriented fashion.
|>>    Here`s the problem description :
|>>        There is an object class called Holder. The object holder can hold various
|>>        things : Pencils, Pens, Erasers etc. I`ve declared different classes for
|>>        these things ( class PENCIL, class PEN, class ERASER etc. ).
|>>
|>>        The user of this program will choose at the runtime what objects to put
|>>        in the holder. The problem is how do i store this information (i.e the
|>>        contents ) in class HOLDER. I cannot create a single array of objects
|>>        because each object is of different class.
|>>
|
|>I assume that it reasonable to allow Holder to hold any one of these objects
|>because there is some similarity between these objects.  e.g., they are all
|>drawing tools or whatever.  This concept of similarity should be captured by
|>classifiying each of these item classes as subclasses (or specializations) of
|>the common class (e.g., drawing_object - which may be abstract).  Then, the
|>Holder class simply has to be declared to be able to hold objects of type
|>drawing_object.  Then, any instance of PENCIL, PEN, ERASER and any other class
|>derived from drawing_object cam be held by a Holder instance.  Aggregates can
|>hold heterogeneous collections etc.
|
|I've been following this thread with some interest, and I find your reply
|interesting.
|
|I agree with your analysis of the problem re: subclasses, however, what if
|you buy/purchase/steal classes from multiple vendors/coworkers/victims ?
|They necessarily will not have a common base class.  (Or is this correctly
|deemed "superclass?")
|
|Borland does this - they have a common base class, "Object", as I recall, and
|all of the storage classes can hold 'objects'.  This allows the storage
|classes to hold just about anything in their provided class library, as every-
|thing is derived from the 'object' class.
|
|Any ideas/thoughts from anyone on how to handle dissimilar base classes in a
|'holder' class?

This is the primary legimate use of multiple inheritance.

    class Grandfathered : public Object, public ThirdPartyThing
    {
    ...
    };

Watch out for member function name conflicts.

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
It's not the way that you say it    |
when you do those things to me,        |    -Richard Hartman
it's more the way you really mean it    |    [e-mail]
when you tell me what will be.        |