NetNews Usenet Archive 1992 #18/NN_1992_18.iso/spool/comp/lang/cplus/12324

Text File | 1992-08-13 | 2.9 KB | 76 lines

Newsgroups: comp.lang.c++
Path: sparky!uunet!newsgate.watson.ibm.com!news.ans.net!cmcl2!HARRISON.CS.NYU.EDU!osinski
From: [e-mail] (Ed Osinski)
Subject: Re: 'virtual' static members
Message-ID: <[e-mail]>
Sender: [e-mail] (Mr Background)
Reply-To: [e-mail]
Organization: New York University
References: <[e-mail]>
Date: Thu, 13 Aug 1992 21:15:47 GMT
Lines: 63

In article <[e-mail]>, [e-mail] (Ralph Melton) writes:
|>
|> I am trying to write a general enumeration class that can do output
|> of itself.  If I was doing it as a single class, part of it might
|> look like
|>
|> class myEnum
|> {
|>   int value;
|>   static int names;
|> friend ostream& operator<< (ostream&, myEnum&);
|> //...
|> }
|>
|> ostream& operator<< (ostream& os, myEnum& e);
|> { return ( os << e.names[e.value] ); }
|>
|>
|> This is all well and good, but I would like to make this into an
|> abstract base class, so that for a specific type of MyEnum, you subclass
|> myEnum, and give the subclass its own static "names" array.
|> I would like to leave the operator<< function the same, if possible.
|>
|> Ideally, I would like to be able to define a fully functional
|> subclass of myEnum in just one or two lines.
|>
|> Can this be done?  If I don't declare the array in myEnum, this
|> operator<< function can't see it; if I do declare it, I can't
|> override it in subclasses.
|>
|> I think I can see more or less skanky ways around this problem using
|> macros or specialized constructors; I am curious as to whether there
|> is any way to declare a static member that is different for every
|> subclass.

No there isn't, but you can get the same effect by declaring static data
members in each class to hold the table, and defining a virtual member function
that returns that class's table.  In your print function, instead of referring
to "names", you would refer to "enum_obj.get_names()".  This will require each
class to have:

    - a new static data member for the table (the variable name can be
          the same in each class)
        - an initialization of that static data member (which must be done
      outside the class)
    - a redefinition of the "get_names" function which simply returns the
          static data member specifying the table of that class

|>
|>
|> Ralph Melton
|> --
|> Ralph Melton            [e-mail]
|> "Then ye shall know the truth, and the truth shall make you free."

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