Path: utzoo!utgpu!water!watmath!clyde!rutgers!mcnc!thorin!grant!haskell
From: haskell@grant.cs.unc.edu (Phyllis Haskell)
Newsgroups: comp.lang.c++
Subject: use of overloaded operators with derived classes?
Message-ID: <656@thorin.cs.unc.edu>
Date: 12 Jan 88 17:58:37 GMT
Sender: news@thorin.cs.unc.edu
Lines: 49


We're having a problem trying to use overloaded operators with derived classes.
A sample program follows in which we are attempting to pass a derived class to
the routine "test", which we want to multiply the instances of any derived 
class.  We want "test" to be a generic routine which does not need to "worry"
about the types of arguments used in its invocation.  Is there a way to do this?


#include <stdio.h>
class Datatype
{
  public:
    virtual Datatype* newobj() {fprintf (stderr,"newobj for base\n");}
    virtual void dump() { fprintf (stderr," dump undefined for base class\n");}
};

class Dint: public Datatype
{
  int temp;
  public:
    Dint () {}
    Dint (int in) {temp = in;}
    Dint& operator* (Dint& in) 
    { 
      Dint& newvar = *new Dint;
      newvar.temp = temp * in.temp;
      return newvar;
    }
    Datatype* newobj() {fprintf (stderr,"newobj for Dint\n");
			return new Dint;}
    void dump () { fprintf (stderr, "Dint = %d\n", temp); }
};

void test(Datatype& in, Datatype& in2)
{
  Datatype* temp  = in.newobj();   // create an instance of a subclass ??
/* 
   (*temp) = in * in2;      // here's our problem - how do we get proper 
                            // * operator for subclass. (commented to prevent
                            // compiler error)
*/
  temp->dump();
}

main ()
{
  Dint inta(5), intb(6);
  test(inta, intb);
}