Path: utzoo!attcan!uunet!microsoft!ilanc
From: ilanc@microsoft.UUCP (Ilan CARON)
Newsgroups: comp.lang.c++
Subject: Re: Multiple Inheritance
Message-ID: <58411@microsoft.UUCP>
Date: 20 Oct 90 20:23:35 GMT
References: <9528@milton.u.washington.edu>
Reply-To: ilanc@microsoft.UUCP (Ilan CARON)
Distribution: na
Organization: Microsoft Corp., Redmond WA
Lines: 67

In article <9528@milton.u.washington.edu >  you write:
	[I've edited the classes, leaving the important stuff...]
 >  
 > class Point
 > {
 > 	public:
 > };
 > 
 > class PhysicalPoint : Point
 > {
 > 	public:
 > 		PhysicalPoint(short x, short y);
 > };
 > 
 > class LogicalPoint : Point
 > {
 > 	public:
 > 		LogicalPoint();
 > 		LogicalPoint(double wx, double wy);
 > }
 > 
 > class Rectangle
 > {
 > 	Rectangle(Point *ul, Point *dr);
 > }}
 > 
 > 
 > 	Now with the above definitions I should be able to do this (in TC++)    
 > right?
 > 	
 > 	LogicalPoint *lPoint = new LogicalPoint(10.0, 10.0);
 > 	LogicalPoint *lPoint2 = new LogicalPoint(30.0, 30.0);
 > 
 > 	PhysicalPoint *pPoint1 = new PhysicalPoint(10, 10);
 > 	PhysicalPoint *pPoint2 = new PhysicalPoint(30, 30);
 > 
 > 	Rectangle lRect(lPoint1, lPoint2);
 > 	Rectangle pRect(pPoint1, pPoint2);
 > ---------------------
 > bytor@milton.u.washington.edu

Hi,

Well, one of the problems in your definitions (aside from some minor
typos) is that Rectangle::Rectangle() is private (that's C++'s default
accessibility.  One effective way (and in this case unintentional)
way of making a class "non-instantiable" by clients is to make its
constructors private.

Another problem is that LogicalPoint and PhysicalPoint derive
*privately* from Point (again this is the default), which I'm sure
you did not intend.  Use e.g. class LogicalPoint : public Point {...};

The effect of the latter is that a pointer to a LogicalPoint
(e.g. lPoint1) can't be cast to a pointer to a Point.  Thus the
following fails:
	Rectangle lRect(lPoint1, lPoint2);
for two reasons:

	(1) The constructor Rectangle::Rectangle(Point*, Point*) is private.
and
	(2) LogicalPoint derives privately from Point, hence can't
	cast lPoint1 and lPoint2 to Point*.

Hope this helps.

--ilan caron (microsoft!ilanc@beaver.cs.washington.edu)