| Re: IDL objects (not object graphics) tutorial? [message #46546 is a reply to message #46457] |
Thu, 24 November 2005 01:04   |
b_gom
Messages: 105
Registered: April 2003
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Senior Member |
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Hi Richard,
Not knowing what your background is, I'll aim low..
Richard G. French wrote:
> I've scoured the web in vain
> looking for a simple tutorial on how and when to use objects in IDL. I've
> found a few generic tutorials praising the virtues of object-oriented
> programming, but almost none of the examples give me any sense of why one
> would go to the trouble.
'How' is covered reasonably well in the IDL manuals. 'When' is another
question altogether.
> What I am looking for is something with a simple application or two in which
> it is both clear why using objects is superior AND which explains what is
> meant by self and methods and classes. Without some specific examples to
> look at, I am having a hard time making sense of the nomenclature or of the
> value of the approach.
Let me try to summarize in a simple-mined, 1:00 AM kind of way:
Think of what problem you want to solve. If it is something like
'calculate such-and-such' or 'perform operation X on an array', then
you probably want a simple function of procedure. If you can think of
your problem as if it were some sort of abstracted machine or 'object'
(like a kitchen appliance or a tree or something), then you probably
want to make an Object.
Most IDL widget applications fall into the latter category. Although
they can almost always be written using non-OOP techniques, there are
often times when the little extra effort to write them as objects pays
off. It's hard to actually get a feel for this without trying both
techniques first.
Here are two excercises that should illustrate some advantages of OOP:
Q) write a program that acts like a stack of 'things', where a thing is
any IDL variable or type. The program should allow you to add things to
the stack, pop them off the stack, tell you how many things are on the
stack, etc. Also you need to be able to have multiple different stacks
of things all co-existing at the same time.
A) Look at the 'linkedlist' object on David Fannings webpage for an
object-oriented solution. If you come up with a non-object-oriented
solution, let me know.
Q) write a program that acts like a dynamic plot of some data. You need
to be able to add lines to the plot, remove them, zoom, change colors,
annotate, etc, and all interactively at run-time. Also, your program
needs to be simple enough so that other people can call it from their
program with a few simple lines of code, and have serveral plots open
at one time.
A) Try the non-OOP approach for 15 minutes, then look at David's site.
There's probably an object there that has most of what you need, and in
15 minutes you can probably add on a few custom methods to extend the
functionality to do the rest. Chances are, after a few years, you'll
still be adding features to your object and it will do just about
everything you can think of. But, here's the key: after all these
changes, the other programs you wrote that call the object years ago
will still work, and you or others will still be able to extend the
object further or adapt it for other uses.
> Someone must be out there just waiting to get rich writing a book on this
> topic.
Back in 1999, a good general introduction to OOP was being written up.
It's now available here:
http://mindview.net/Books/TICPP/ThinkingInCPP2e.html
This is for C++, but the general principles apply to IDL.
Brad
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