Path: utzoo!utgpu!watserv1!watmath!att!dptg!pegasus!hansen
From: hansen@pegasus.ATT.COM (Tony L. Hansen)
Newsgroups: comp.lang.c++
Subject: Re: How to use Inheritance instead of nesting classes
Summary: use multiple inheritance with a pair of intermediate classes
Keywords: Inheritance, class
Message-ID: <4916@pegasus.ATT.COM>
Date: 22 Jul 90 19:24:03 GMT
References: <13647@shlump.nac.dec.com>
Reply-To: hansen@pegasus.ATT.COM (Tony L. Hansen)
Organization: AT&T Bell Labs NJ USA
Lines: 39

< I have some entities, viewports to be specific, that must be able to
< reside multiple doubly linked lists at once.  In otherwords, a view port
< might be both in  the "mouse button depress queue" and in the "mouse
< motion queue" at the same time.

< For an entity to reside on a single list I could follow Bjarne's example
< in his book just fine.  But how do I define an object that resides on
< multiple lists and use inheritance?  Can I inherit the same object twice
< in the same class?

The method which occurred to me several years ago when someone else asked me
the same question was to use multiple inheritance after introducing a pair
of intermediate classes which are used to differentiate between the two
lists.

Something like this should do the trick:

	class mouse_button_link : public slink
	    { ... };

	class mouse_button_queue : public slist
	    { ... };

	class mouse_motion_link : public slink
	    { ... };

	class mouse_motion_queue : public slist
	    { ... };

	class viewport :
		public mouse_botton_link,
		public mouse_motion_link
	    { ... };

Of course, this requires suitable definitions of slink and slist.

					Tony Hansen
				att!pegasus!hansen, attmail!tony
				    hansen@pegasus.att.com