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From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: comp.lang.c++
Subject: Re: Novice question re: class derivation
Message-ID: <10354@alice.UUCP>
Date: 19 Jan 90 14:07:43 GMT
References: <44.UUL1.3#5109@pantor.UUCP>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 74

In article <44.UUL1.3#5109@pantor.UUCP>, richard@pantor.UUCP (Richard Sargent) writes:

> 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?

I don't see anything wrong with what you've done so far.
You're missing just one little trick.  The idea is to never
use the object directly.  Rather, refer to it through a
pointer or reference to a base class object.

For example:

	class Display { /* ... */ };
	class ColorDisplay: public Display { /* ... */ };
	class HighResDisplay: public Display { /* ... */ };

Somewhere you have to make a decision on what kind of display you're
using.  Probably the most direct way to do it is something like this:

	Display* dp;

	if (cond1)
		dp = new ColorDisplay;
	else if (cond2)
		dp = new HighResDisplay;
	else /* ... */

Now you code all display operations in terms of pointer operations
on dp

	dp->clear();
	dp->draw(image);
	// and so on

When you're finally done, you can free it:

	delete dp;

but for this to work, your Display base class must have a virtual
destructor.

Perhaps a better way of structuring it, if you can, is to do all the
real work in a subroutine that takes the Display object as a
formal parameter.  Then the allocating and freeing are done in one
routine and everything else is done somewhere else.

If you do this, you can make that parameter a reference to a Display
rather than a pointer to a Display, which you may find more natural:

	void MyApplication(Display& d)
	{
		// ...
		d.clear();
		// and so on
	}

	main()
	{
		if (cond1) {
			ColorDisplay d;
			MyApplication(d);
		} else if (cond2) {
			// ...
		} // ...
	}

Now you don't have to allocate and free Display objects on the
free store -- they're local to the various branches of the `if'.

Hope this helps.
-- 
				--Andrew Koenig
				  ark@europa.att.com