Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!wuarchive!udel!rochester!kodak!islsun!cok
From: cok@islsun.Kodak.COM (David Cok)
Newsgroups: comp.lang.c++
Subject: Re: typesafe downward casting
Message-ID: <1991Apr5.150148.21840@kodak.kodak.com>
Date: 5 Apr 91 15:01:48 GMT
References: <18.UUL1.3#8618@softrue.UUCP> <1991Apr2.161809.16952@kodak.kodak.com> <27FA1EC4.511C@tct.com>
Sender: cok@Kodak.COM
Organization: Eastman Kodak Co., Rochester, NY
Lines: 34

In article <27FA1EC4.511C@tct.com> chip@tct.com (Chip Salzenberg) writes:
>According to cok@islsun.Kodak.COM (David Cok):
>>I would maintain ... that a good program generally has few casts and a
>>good programming language allows most programs to be written without casts.
>
>Indeed.  And that is true of C++ today -- IF you are willing to avoid
>using Base* all over the place when Derived* is meant.  (Template
>collections are a tremendous help.)
>
>In other words, the way to regain an object's lost static type is not
>to lose it in the first place.
>-- 

I am certainly looking forward to having an environment that supports templates
(no such luck here yet), and think it will do great things for reusing 
implementations of Lists, etc., but I don't think it is a replacement for
inheritance.  

If you use inheritance simply to provide a new implementation of a class, you
can keep manipulating the Base* values, and never need to deal with the
Derived class at all except to state in a declaration which derived class to
use, e.g.  List* list = new MySpecialListImplementation.

But another use of inheritance is to add functionality to an existing class by
deriving from it.  Now virtual functions which returned Base* will still
return Base* in the derived class -- so I cannot help losing the object's
static type.  In this context one must either have contravariance on the
return type of virtual functions or type-safe down casting -- or resort to
current C++ which provides only type-unsafe down casting or corrupting the base
class with the interface to the added functionality of the derived class.

David R. Cok
Eastman Kodak Company -- Imaging Science Lab
cok@Kodak.COM