Path: utzoo!attcan!uunet!cs.utexas.edu!tut.cis.ohio-state.edu!ucbvax!hplabs!hplabsz!bs
From: bs@hplabsz.HPL.HP.COM (Bob Shaw)
Newsgroups: comp.lang.c++
Subject: Return types of virtual member functions
Message-ID: <3403@hplabsz.HPL.HP.COM>
Date: 27 May 89 01:01:19 GMT
Organization: Hewlett-Packard Laboratories
Lines: 94


I'm trying to build a "bushy" single inheritance tree.  Many of the child
classes define a few member functions beyond those of the parent.  Most of 
the system deals with the various objects at the highest level of 
abstraction (i.e. only use the parent member functions), but parts of the
system change the level of abstraction and deal with the object as an
instance of a child class.

My question involves the return value types in virtual member functions.
If a parent class defines a virtual member function such as

 virtual parent* msg();

I would like a child (derived) class to be able to define its version of the
member function as

 virtual child* msg();     rather than    virtual parent* msg();.

It's my understanding that a child* can always be assigned to something
defined as a parent*, so I can't see why this should break the type system.
Obviously, children of such a child would be constrained to return a pointer
to child or something derived from it.

The code below does not work as I had hoped.  Can someone involved in the
ATT C++ 2.0 release tell me if this will work in 2.0 and, if not, whether
it's a bug or why it isn't a reasonable thing to do?


Thanks,
bs           <- obviously not the att one


----------------------------
Output from following code:


parent->msg()
  parent::msg entered

child->msg()
  child::msg entered

child_as_parent->msg()
  parent::msg entered       <---- I'd hoped for "child::msg entered"

----------------------------
#include <stream.h>

class parent
{
public:
  virtual parent* msg();
};


class child : public parent
{
public:
  virtual child* msg();
};


parent* parent::msg()
{
  cout << "  parent::msg entered\n";
  return this;
}


child* child::msg()
{
  cout << "  child::msg entered\n";
  return this;
}

void main()
{
  parent* presult;
  child*  cresult;

  parent* p    = new parent;
  child*  c    = new child;

  cout << "\nparent->msg()\n";
  presult = p->msg();

  cout << "\nchild->msg()\n";
  cresult = c->msg();

  cout << "\nchild_as_parent->msg()\n";
  p = c;
  presult = p->msg();

}