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Re: global variables in IDL [message #28638 is a reply to message #28637]

Fri, 28 December 2001 08:22

John-David T. Smith is currently offline

John-David T. Smith
Messages: 384
Registered: January 2000

Senior Member

Pavel Romashkin wrote:
>
> I agree with Kristine. Working around Commons writing a global variable
> code, if you use Xmanager, is kind of silly becasue Xmanager uses Common
> blocks in the first place.

This objection, though oft-repeated, is somewhat unfounded, I think.
The chief problem with common blocks for global state data is that only
a single such block exists. For example, if you store the widget id of
a button in:

common mywidget, mybutton
mybutton=widget_button(base,value='Foo')

Then `mybutton' will be the *only* button that can exist for your
program in a single IDL session. You won't be able to invoke your
program more than once at a time. Actually, you will, and that single
button variable slot will get overwritten, and very bad things will
happen.

XManager does use common blocks, but, by design, it uses them
intelligently. Instead of overwriting common block variables, it
maintains global lists of all the widgets it needs to manage, extending
and pruning the lists as necessary. It's an example of common block
usage which doesn't prevent multiple copies of a single widget program
from running. (Here I gloss over the subtlety that widget programs run
with /NO_BLOCK are actually handled by the command-line loop code, and
not XManager at all.)

The fact that common blocks, in their basic usage, can cause this subtle
problem doesn't imply that this problem is endemic to their use:
XManager is an example of a more intelligent setup which capitalizes on,
rather than suffers from, the single global instance status of common
block variables.

JD

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[Message index]

		Re: global variables in IDL By: Pavel A. Romashkin on Fri, 28 December 2001 08:39
		Re: global variables in IDL By: John-David T. Smith on Fri, 28 December 2001 08:22
		Re: global variables in IDL By: Pavel A. Romashkin on Wed, 26 December 2001 21:02
		Re: global variables in IDL By: Kristine Hensel on Wed, 26 December 2001 17:39
		Re: global variables in IDL By: Gert on Wed, 26 December 2001 17:02
		Re: global variables in IDL By: David Fanning on Tue, 25 December 2001 09:25

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