Path: utzoo!censor!geac!becker!pantor!richard
From: richard@pantor.UUCP (Richard Sargent)
Newsgroups: comp.lang.c++
Subject: Novice question re: class derivation
Message-ID: <44.UUL1.3#5109@pantor.UUCP>
Date: 18 Jan 90 15:24:11 GMT
Organization: Pansophic Systems Inc, Graphics Product Company
Lines: 55

We have encountered a problem designing a tree of classes, that I
hope someone out there can help resolve. I'm sure that it is just
a case of looking at the problem "the wrong way", but I can't see
the right way.

[Apology: I'll use some PC terminology in the description, but the
platform and hardware is not the problem, per se.]

We want to be able to provide a base class called Display, which has
derived classes for specific types of displays (such as Vista, Targa,
VGA, raster file, etc.). Our first pass results in the following
tree:

                  Display
                     |
      +--------+-----+--------+--------+
      |       |      |        |        |
    Vista   Targa   VGA   RasterFile  etc.

Now the problem comes when we try to code for the display device.
Ideally, we want to be able to use an instance of class Display,
so that our code is *written* device independent. We would like
to simply code "Display device" and apply operations to "device",
without concern for just which device it really is. The thinking
is the constructor for class Display is able to determine what
device it present and construct the appropriate class.

An alternative, but less elegant solution, is to code something
like the following, but I don't even know if this is valid:

        switch (inq_device_type()){
           case dv_VISTA:
              Vista device;
              break;
           case dv_TARGA:
              Targa device;
              break;
           case dv_VGA:
              VGA   device;
              break;
           .
           .
           .
           }


So the bottom line is: How can I set up a hierarchy of device
classes that will allow me to code (my main application, at least)
to be independent of the display device?

All help is appreciated. If any clarifications are needed, please
ask. Thank you.

Richard Sargent                   Internet: richard@pantor.UUCP
Systems Analyst                   UUCP:     ...!mnetor!becker!pantor!richard