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From: asc@concurrent.co.uk (Andy Chittenden)
Newsgroups: comp.lang.c++
Subject: problem with virtual inheritence
Message-ID: <952@sl10c.concurrent.co.uk>
Date: 14 Dec 89 12:45:36 GMT
Sender: info@concurrent.co.uk
Reply-To: asc@concurrent.co.uk (Andy Chittenden)
Organization: Concurrent Computer Corp (ESDG), Slough, U.K.
Lines: 66

The attached code demonstrates a problem when using C++ v2 virtual
inheritence.  We have a Collection class that can hold pointers to
instances of class Object (or derived from Object).  In the example, I
have declared two collections.  One is a collection of Widgets and the
other is a collection of OtherWidgets.  I can use my collection of
Widgets as a collection of Widgets.  I can add Widgets to the collection
and I can get Widgets out of it.  However, I can only use my
collection of OtherWidgets as a collection  of Objects.  I can add
OtherWidgets to the collection but can only get Objects out of it as
OtherWidget virtually inherits from Object even though I only add
OtherWidgets to it.  Thus if I use virtual inheritence, I must
reimplement my collection to be a collection of OtherWidgets.  I have
thus lost a lot of my code reuse. 

Has anyone else discovered this problem and if so, have they found a way
out of it?

Rgds, Andy Chittenden

Program follows:

class Object {
public:
  Object();
};

class Collection : public Object {
public:
  Collection();
  unsigned long add_member(const Object* new_member); 
  		// returns pos of newly added member
  Object* get_Nth(unsigned long position);
};

class Widget : public Object {
public:
  Widget();
};

class OtherWidget : public virtual Object {
public:
  OtherWidget();
};

main()
{
  Widget x_widget;
  Collection x_widget_collection;
  x_widget_collection.add_member(&x_widget);

  Widget* widget_ptr;

  widget_ptr = (Widget *) x_widget_collection.get_Nth(1);

  OtherWidget x_otherwidget;
  Collection x_other_widget_collection;
  x_other_widget_collection.add_member(&x_otherwidget);

  OtherWidget* other_widget_ptr;

  other_widget_ptr = (OtherWidget *) x_other_widget_collection.get_Nth(1);
  			// does not compile
}

"coll.cxx", line 40: error: cast: Object* ->derived OtherWidget*; Object is virtual base
1 error