Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!pdn!tct!chip
From: chip@tct.com (Chip Salzenberg)
Newsgroups: comp.lang.c++
Subject: Re: About Lists and things...
Message-ID: <28692A4A.59B7@tct.com>
Date: 26 Jun 91 23:59:06 GMT
References: <2865E7A8.179A@tct.com> <19131@prometheus.megatest.UUCP>
Organization: TC Telemanagement, Clearwater, FL
Lines: 29

According to djones@megatest.UUCP (Dave Jones):
>> ...   Never design code that requires downcasts.
>>         Or, in other words: if the static type is important, why worry
>>         about regaining it?  Don't lose it in the first place.
>
>Something tells me this has all been hashed over previously. The "static
>type" was not "lost" in the first place. It did not exist in the first place.

By "static type" I mean "compile-time type".  Perhaps an example will
clarify my point.

If you put a Circle into a ShapeList, you lose the Circle's
compile-time type, because ShapeList.first() returns |Shape*|.
Suppose that you decide to call a Circle-specific function on a Circle
stored in a ShapeList.  Some would cast the |Shape*| to a |Circle*|
and carry on.  My coding practice would require you to replace the
ShapeList with a CircleList.

Of course, that change may not be practical, perhaps because there are
other non-Circle objects in the ShapeList.  If so, congratulations!
You have just discovered that the function in question is actually a
Shape function in disguise.  You should therefore add it to the Shape
interface, presumably as a new virtual function.

Clearer now?
-- 
Chip Salzenberg at Teltronics/TCT     <chip@tct.com>, <uunet!pdn!tct!chip>
 "I want to mention that my opinions whether real or not are MY opinions."
             -- the inevitable William "Billy" Steinmetz