NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/sys/amiga/programmer/2797

Text File | 1996-08-05 | 1.4 KB | 55 lines

Path: columba.udac.uu.se!chaph!t94dwi
From: [e-mail] (Daniel Widenfalk)
Newsgroups: comp.sys.amiga.programmer
Subject: Return types and virtual methods.
Date: 5 Feb 1996 11:40:47 GMT
Organization: Uppsala University
Message-ID: <4f4qbv$[e-mail]>
NNTP-Posting-Host: chaph.tdb.uu.se
X-Newsreader: TIN [version 1.2 PL2]

Hi all,

I was trying to create a "set" class this weekend. I tried to do this:

class Set {
    public:
    virtual    ~Set();

    virtual Set & operator=(const Set &) = 0;
    virtual Set operator~() = 0;
    ...
};

class BitSet : public set {
    public:
        ~BitSet();

    BitSet & operator=(const BitSet &);
    BitSet operator~();
    ...
};

As you can see, I want to be able to handle several diffrent types
of sets the same way. Now to the problem. This doesn't work. I get
the error-message in my .cc file:

BitSet operator~();
BitSet cannot create instance of abstract object. (Or something like it)

As I see it, I get two assignment operators. One for Set & and one for
BitSet &. This make the Set & one undefined in BitSet and therefore
BitSet is an abstract class. This makes the compiler barf when it gets
to operator~, which
1) Tries to return an abstract class instance
and
2) HAVE DIFFRENT return values.

Now, my question is. How do I fix this? I want to be able to have several
diffrent representations of sets, but I want them to look the same to the
user.

All help appreciated
/Daniel Widenfalk
[e-mail]