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From: barmar@think.com (Barry Margolin)
Newsgroups: comp.lang.c++
Subject: Re: asking an object for its type
Message-ID: <1991Feb19.023528.29494@Think.COM>
Date: 19 Feb 91 02:35:28 GMT
References: <23984@netcom.COM> <1190@sheol.UUCP> <1991Feb19.000449.22255@gpu.utcs.utoronto.ca>
Sender: news@Think.COM
Organization: Thinking Machines Corporation, Cambridge MA, USA
Lines: 35

In article <1991Feb19.000449.22255@gpu.utcs.utoronto.ca> craig@gpu.utcs.utoronto.ca (Craig Hubley) writes:
>Arg.  A totally contextual algorithm.  I do NOT want to write a special
>function for all shapes to respond to this.  What I would really like to
>do is something like:
>	"if x.type=circle then if x.diameter > 30 then return x"

Even in languages that provide a way to do that, you probably wouldn't want
to write it that way, because it doesn't handle derived classes properly.
For instance, if square is derived from regular_polygon and quadrilateral,
x.type can't be all three.

What you want is something like

	if x.is_circle() ...
or
	if x.is_of_type("circle") ...

The latter can be implemented by writing something like

int circle::is_of_type(char *type)
{
	return (strcmp(type, "circle") == 0) || shape::is_of_type(type);
}

int square::is_of_type(char *type)
{
	return (strcmp(type, "square") == 0) ||
	  quadrilateral::is_of_type(type) ||
	  regular_polygon::is_of_type(type);
}
--
Barry Margolin, Thinking Machines Corp.

barmar@think.com
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