| Using NO_COPY with pointers [message #34733] |
Mon, 14 April 2003 08:22 |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Folks,
This may be common knowledge, but I wasn't aware of it, and
it is one of those things that makes you feel all warm and
goose-pimply about IDL.
I was adding a "user value" to all of my objects
today, via a UVALUE field in the "atom" object that
is inherited by all objects in my library. This field
is, of course, a pointer.
Naturally enough, I want to be able to get and set
the "value" of this field sometimes without making
a copy of the data. This is no problem when I am
adding the information to the pointer, I simply use
the NO_COPY keyword on PTR_NEW:
self.uvalue = Ptr_New(uvalue, /No_Copy)
But it is a bit of a problem when I want to "get"
the value back:
uvalue = *self.uvalue
I was of the impression that pointer de-referencing
*always* made a copy of the data. But on a whim, I
tried this:
IF Keyword_Set(no_copy) THEN uvalue = Temporary(*self.uvalue)
Lo and behold, this did *exactly* what I wanted it to do!!
Help, *self.uvalue
<PtrHeapVar2093>
UNDEFINED = <Undefined>
Moreover,
Print, Ptr_Valid(self.uvalue)
1
Perfect! A valid pointer that points to an undefined variable.
Hooray! You gotta love IDL! :-)
Cheers,
David
--
David W. Fanning, Ph.D.
Fanning Software Consulting, Inc.
Phone: 970-221-0438, E-mail: david@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Toll-Free IDL Book Orders: 1-888-461-0155
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