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Re: Creating pointer in structure [message #27737 is a reply to message #27731]

Thu, 01 November 2001 12:57

John-David T. Smith is currently offline

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

Senior Member

"K. Bowman" wrote:
>
> If I need to define a structure containing a pointer before I know the
> characteristics of the associated heap variable, which of the following
> is preferable? Does it make any difference, or is it simply a matter
> of programming taste?
>
> For example:
>
> a = {point: PTR_NEW()} ;Create struct w/ null pointer
> ... figure out what n is
> a.point = PTR_NEW(FINDGEN(n)) ;Replace null pointer
>
> or
>
> b = {point: PTR_NEW(/ALLOCATE_HEAP)} ;Create struct w/ pointer->undef
> ... figure out what n is
> *b.point = FINDGEN(n) ;Define heap var
>
> Thanks, Ken

I use both methods. When I'd like ptr_valid() to be the ultimate test
of whether anything is in the slot, I use the former. For lists of
variable length, including 0, this is an excellent test, since it
doesn't rely on what precisely is in the slot. It does, however, force
you to check before dereferencing your pointer.

Even an undefined variable could get stuck the slot to indicate
something is there. It's unusual, but you could end up with:

foo=1 & boo=temporary(foo)
a.point=ptr_new(foo)

which is the same really as a.point=ptr_new(/ALLOCATE_HEAP). In this
case:

IDL> print,n_elements(*a.point)
0
IDL> print,ptr_valid(a.point)
1

On the other hand, when I have a list which will always have at least
one element, I often use the latter method, to remind me of the fact.
Then you can dereference freely, and use n_elements() to see just what
is there.

As long as you remember the difference, you'll be fine.

One other case where having true null pointers is useful: when saving
objects or structures, it's sometimes useful to kull out useless bits
(like widget states and id's), which aren't really appropriate for
keeping on disk. If you've stuck them behind a pointer, you can just do
a:

state=self.myuselesswidgetstate
self.myuselesswidgetstate=ptr_new()

before saving and restore it afterwards.

JD

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[Message index]

		Re: Creating pointer in structure By: hcp on Fri, 02 November 2001 08:31
		Re: Creating pointer in structure By: Ron Syml on Fri, 02 November 2001 08:02
		Re: Creating pointer in structure By: K. Bowman on Fri, 02 November 2001 07:21
		Re: Creating pointer in structure By: John-David T. Smith on Thu, 01 November 2001 14:49
		Re: Creating pointer in structure By: Pavel A. Romashkin on Thu, 01 November 2001 14:19
		Re: Creating pointer in structure By: John-David T. Smith on Thu, 01 November 2001 12:57
		Re: Creating pointer in structure By: Pavel A. Romashkin on Thu, 01 November 2001 08:56
		Re: Creating pointer in structure By: Paul van Delst on Thu, 01 November 2001 07:07
		Re: Creating pointer in structure By: David Fanning on Thu, 01 November 2001 07:01

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