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From: pkron@corona.com (Peter Kron)
Organization: Corona Design, Inc., Seattle, WA
Reply-To: pkron@corona.com
Distribution: world
Date: Fri, 5 Aug 1994 06:04:45 PDT
Message-ID: <1994Aug05.130445.352@corona.com>
Newsgroups: comp.std.c++
Subject: Re: Multiple inheritance and delete
Lines: 41
References: <776024044snz@storcomp.demon.co.uk>

From: Philip@storcomp.demon.co.uk (Philip Hugh Hunt)
>  One example follows:
>  
>  class Point {
>  public:
>     int x;
>     int y;
>     Point(int xx =0, int yy =0) { x=xx; y=yy; };
>     Point operator+(Point p) {return Point(x+p.x, y+p.y);};
>  };

class ConstrainedPoint : Point
    {
public:
    int limit;
    Point operator+(Point p) 
        {return Point( x+p.x < limit ? x+p.x : limit, y+p.y)};
    }

If this class is used in a collection of Point, it is probably going
to cause some subtle bugs. It's probably guaranteed to happen in the
maintenance of any software using Point. Since this use--which seems
reasonable enough--wasn't anticipated by the designer of Point, the
reuse of Point is limited. Why should this be the default?

>  If this class was forced to be polymophic, it would be
>  less efficient.  The loss of efficiency would probably
>  be unacceptable if this class was used in eg a windowing
>  system.

I could certainly produce a benchmark supporting your claim. I could
also show that using a simple C struct would improve performance
further. However, in a real system, I'd be more concerned about the
total efficiency. If a polygon of a 1000 points is translated using
operator+, which will contribute more to the overall performance:
memberfunction calls or rerendering the polygon following translation?
---
NeXTMail:peter.kron@corona.com
Corona Design, Inc.
P.O. Box 51022
Seattle, WA 98115-1022