Path: utzoo!attcan!uunet!samsung!munnari.oz.au!brolga!uqcspe!batserver.cs.uq.oz.au!iain
From: iain@batserver.cs.uq.oz.au (Iain Fogg)
Newsgroups: comp.lang.c++
Subject: Multiple inheritance and type casting
Message-ID: <5087@uqcspe.cs.uq.oz.au>
Date: 3 Oct 90 02:02:12 GMT
Sender: news@uqcspe.cs.uq.oz.au
Reply-To: iain@batserver.cs.uq.oz.au
Lines: 68


Assume the following class definitions:

class B {
  .
  .
  .
  public:
    virtual f ( B &b );
  .
  .
  .
}


class D1 : virtual public B {
  .
  .
  .
  public:
    virtual f ( B &b );
  .
  .
  .
}


class D2 : virtual public B {
  .
  .
  .
  public:
    virtual f ( B &b );
  .
  .
  .
}


class X : public D1, public D2 {
  .
  .
  .
  public:
    virtual f ( B &b );
  .
  .
  .
}


If D1::f() cast the formal parameter to type D1 (ie. (D1 &) b), for
example to interrogate private data members, no problems. However, if
X::f() casts to an X& my compiler complains (TC++ V1.0) saying
"Cannot cast from B& to X&". I *know* that when X::f() is called, the
actual parameter is an X&. Why is it so? Is there a fix? Email replies
and I will summarise.


+----------------------------------------------------------------------+
+ Dr Iain Fogg                                                         +
+ Senior Research Officer                                              +
+                                                                      +
+ Department of Computer Science              Telephone: (07) 377 2903 +
+ University of Queensland                          Fax: (07) 371 0783 +
+ Queensland                                            Telex: AA40315 +
+ Australia 4072                     email: iain@batserver.cs.uq.oz.au +
+----------------------------------------------------------------------+