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From: nobody@tekecs.TEK.COM (-for inetd server command)
Newsgroups: comp.lang.c++
Subject: Overloading virtual functions with a different return type
Message-ID: <11034@tekecs.TEK.COM>
Date: 17 Feb 89 18:38:01 GMT
Reply-To: brucec@demiurge.GWD (Bruce Cohen)
Organization: Tektronix, Inc., Wilsonville, OR
Lines: 44

I'm using cfront 1.2, and I wan to build a set of classes derived from an
abstract base class which are a kind of "smart pointer", i.e., they can
retrieve the objects they point to from storage somewhere if they are not
currently resident in the program's address space.

The macro solution to generic container classes is not appealing, and I'm
willing to put up with some limitations to avoid it, but one thing disturbs
me: I can't have virtual functions which are declared to return an object
of the type which the derived class manages, even if it's a subclass of the
type managed by the bas class.  For instance:

class Base
{
 public:
     virtual void* Obj() { return obj; }

 protected:
    void* obj;
};

class Derived : public Base
{
 public:
     virtual int Obj() { return (int)obj; }
};

gives me the following message:

 error: virtual Derived::Obj() type mismatch: int Derived::() and void *Base::()

even though void* is in effect an anonymous type, and there is not
really a type mismatch.  Obviously, I can get this example to compile by
making the functions non-virtual, but that removes the nice polymorphism I
wanted to begin with.

Question 1: is this really a language issue, or just a quirk of the current
version of cfront?

Question 2: can anyone suggest a way around this?

Bruce Cohen
Interactive Technologies Division, Tektronix, Inc.
M/S 61-028, P.O. Box 1000, Wilsonville, OR  97070
brucec@demiurge.gwd.tek.com, SOON TO BE: brucec@orca.wv.tek.com