The LA Fox Developer Newsletter

February 1996

Polymorphism (Con’t from page 4) event even though other classes in the hierarchy also have double click code.

Let’s look at this concept another way. You are your parents’ child. However, both you and your parents share common “reactions” to events. For example, if you got hit on the head with a stick-the event-you would have a reaction. If one of your parents got hit on the head with a stick, they would also have a reaction (though they wouldn’t like it either).

Note that this example breaks down a little, since VFP doesn’t support multiple inheritance. That is, you are the descendent of both your parents-you inherit from both of them. Other languages, notably C++, do support multiple inheritance. However, there is some debate about how much multiple inheritance adds to most applications (other than much complexity, that is).

What you might be wondering is, why would polymorphism work any differently than this article is describing? Well, what could happen (and does sometimes in other languages) is that without polymorphism, if you sent a double click message to the child’s form, the base class code would fire (not the child’s). which is not what you want. The fact that polymorphic behavior is automatic(that is, you don’t have to create any special functions for polymorphism

to work correctly as you do in other languages like C++) is a nice benefit of using VFP.

Okay, back to the code above, If you change the CREATEOBJECT to:

oEXAMPLE=CREATEOBJECT("PARENT")

oExample.Show()

READ EVENTS

and then double click the form, you’ll see the WAIT WINDOW for the parent. Sounds good, right? Pretty easy, right? That’s all there is to it!!!

Omitting code in a class

What would happen if we used:

oExample=CREATEOBJECT("CHILD")

/oExample.Show()

READ EVENTS

and we left out the procedure for the DBLCLICKO procedure? Simple. The code for the next member up the

class hierarchy is called. Thus, you’d see the WAIT WINDOW for parent again. If there is no code in the method code for the object you’re double clicking VFP looks up the class hierarchy for method code it can execute. When VFP finds method code for double click up the hierarchy it executes it!

As I hope you see, polymorphism is easy to understand, but hard to spell’

(About the Author: Jim Haentzschel is President of

Hurricane Technology in Alexandria, VA specializing in

FoxPro applications development and in user training.

Jim can be reached at CompuServe 75166,236 and at

(703) 684-1393.)

Out and About (Con't, from page 2)

didn’t receive a newsletter last month. If your dues expired in ‘94, you probably didn’t receive one this month. So everyone make sure you look at your mailing label for membership/expiration status and please respond accordingly. Membership dues are $45, payable to LA Fox, and can be mailed to Allen Garfein, 3449 Beethoven, Los Angeles, CA 90066, or Barry R. Lee, P.O. Box 6624, Huntington Beach, CA 92615-6624, or brought to the next meeting. Dues help defray the cost of the meeting rooms, newsletters, speaker expenses, etc.

Catching Up With Les Les Pinter’s VFP book, The Pinter Visual FoxPro Programming Handbook , has finally hit the shelves of the better bookstores in the area. Expect a review in the next couple of months. And he finally has his forum up on CompuServe. Just key in “GO PINTER”. If you purchase the VFP book, you’ll be able to download all the source code to the new book, as well as past issues of the Pinter FoxPro Letter, from the Pinter Forum.

Elsewhere

For at least the next couple of months, the Orange

County FoxPro Developers Group will have a new

meeting location at AST Research.

Orange County FoxPro Developers Group

AST Research, Inc.

16215 Afton Parkway

Irvine, CA 92713

From the 405, exit at Sand Canyon and go north to Afton Parkway. Proceed east on Alton Parkway to AST Re (Con't, page 8)

Page 5