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Re: Pass by value and performance [message #46707 is a reply to message #46706]

Wed, 14 December 2005 09:02

Paul Van Delst[1]
Messages: 1157
Registered: April 2002

Senior Member

David Fanning wrote:
> Kenneth P. Bowman writes:
>
>
>> True, and I considered that possibility. It would work fine for passing
>> the arrays amongst my own routines, but I have to dereference the
>> pointer to pass the array itself into system routines. Dereferencing a
>> pointer is an expression, is it not?, and expressions are passed by
>> value.
>
>
> Ah, you would be interested to read the new pointer
> section of my book that I just completely yesterday!
> (Well, actually, you could read the damn pointer tutorial
> where I learned about this, too.) Here is the news that
> even I didn't fully appreciate until I put it down in my
> own words. Pointers are just regular IDL variables!!

In the context of this thread (and your example below), shouldn't one say that
/dereferenced/ pointers are just regular IDL variables (i.e. they're not expressions) ??

I don't think I'm splitting hairs here (or am I?)

>
> Consider this function:
>
> PRO JUNK, var
> var = var * 5
> END
>
> And do this:
>
> IDL> a = Ptr_New(5)
> IDL> junk, *a
>
> Here is the question. What is the result of this command?
>
> IDL> Print, *a
>
> The answer might surprise you! :-)

After my recent grokking of objects (thanks to that "object for dummies" website posted
earlier this month), the answer didn't surprise me as much it would of, but it's still a
good reinforcement.

It makes me wonder though - have IDL pointers always behaved that way, or is it a moving
target (ala relaxed structure definitions post IDL 5.3 talked about in another thread) ?

paulv

--
Paul van Delst
CIMSS @ NOAA/NCEP/EMC

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[Message index]

		Re: Pass by value and performance By: JD Smith on Wed, 14 December 2005 11:04
		Re: Pass by value and performance By: David Fanning on Wed, 14 December 2005 09:54
		Re: Pass by value and performance By: Kenneth P. Bowman on Wed, 14 December 2005 09:28
		Re: Pass by value and performance By: David Fanning on Wed, 14 December 2005 09:08
		Re: Pass by value and performance By: Paul Van Delst[1] on Wed, 14 December 2005 09:02
		Re: Pass by value and performance By: David Fanning on Wed, 14 December 2005 07:53
		Re: Pass by value and performance By: Kenneth P. Bowman on Wed, 14 December 2005 07:09
		Re: Pass by value and performance By: peter.albert@gmx.de on Tue, 13 December 2005 23:38
		Re: Pass by value and performance By: Antonio Santiago on Wed, 14 December 2005 22:56

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