| Re: passing object references [message #13468 is a reply to message #13461] |
Wed, 11 November 1998 00:00  |
davidf
Messages: 2866
Registered: September 1996
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Senior Member |
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Philip Aldis (teal@dera.gov.uk) writes:
> I having a bit of trouble with my object references and passing them
> round. I have a feeling it's to do with passing by value and by
> reference, but I haven't had time to test this out yet, so before I
> start ripping my code apart - I'll see if what I suspect is true.
Hang on, hang on. Don't start ripping code just yet! :-)
> I have two widget programs which deal with loading some data in and then
> the second one does an animation. The loader program creates the object
> and loads the data, using the methods and then passes the object with
> its data loaded in to the animator widget program which calls the object
> methods for animating. I pass the object in as a keyword.
>
> Previously the object had been part of the info structure of my loading
> widget program, and then I simply call my animator program
> ANIMATOR, object = info.object
Now, the only things that are passed by reference in IDL are
variables. Everything else, and this includes structure de-references
like we have here, are passed by value. So it is a *copy* of the
object reference that is going into the Animator program, clearly.
But I would argue that that is exactly what you want to happen and,
in fact, is the *point* of heap variables. The reference always points
to the actual object, located on the heap.
> .....fine so far. Now I've tested the object I send, the object I
> receive and they are all valid. Within the main animation program - i.e.
> before the call to xmanager, I've got the user value and tested the
> validity of the object, and yep you guessed it, it's valid. BUT when I
> get the user value in my event handlers, the object is suddenly no
> longer valid.
Then I would like to see how you are storing it in your info structure.
Or, another possibility, did you destroy the first widget program to
run the Animator program? If so, your clean-up routine on the first
program destroyed the object. If this is happening, you may have to
make an actual copy of the object itself and pass that. I just added
a tip on my web page about how to do this:
http://www.dfanning.com/tips/copy_objects.html
> Is it something to do with the fact that I have sent it in a keyword?
I doubt it. If you are trying to get information *back*
from keywords, you can have problems. But that doesn't sound
like what you are doing here.
Cheers,
David
----------------------------------------------------------
David Fanning, Ph.D.
Fanning Software Consulting
E-Mail: davidf@dfanning.com
Phone: 970-221-0438, Toll-Free Book Orders: 1-888-461-0155
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Note: A copy of this article was e-mailed to the original poster.
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