Re: Object Data and pointer assignments

[davidf(at)dfanning.com (David Fanning)]

J.D. Smith (jdsmith@astro.cornell.edu) writes:

> Just to be clear... you are free to free self.inarray, and point it somewhere
> else, at any time.  This can be useful if you have a list which is either empty
> (NULL pointer a.k.a. a dangling reference), or not (pointer to a list of finite
> size).  If the list changes size, and becomes empty again, you can simply free
> it, which indicates its emptiness.  If it then grows again, simply use ptr_new()
> to get another heap variable for it.  So, while it might be easiest in some
> cases only to call ptr_new() once, in other cases it is useful to let a single
> member variable like self.inarray point to different heap variables over its
> life.

Lord knows I need more excitement in my life if I'm quibbling with
quibbles, but let me make one suggestion:

If I want to point to an "empty" variable, I prefer to
use a pointer to an undefined variable. The advantage
to me is that this is a VALID pointer, in contrast
to the NULL pointer, which is an invalid pointer.

Note:

   IDL> a = Ptr_New()
   IDL> Print, Ptr_Valid(a)
         0
   IDL> *a = 5
       % Unable to dereference NULL pointer: A.

   IDL> b = Ptr_New(/Allocate_Heap)
   IDL> Print, Ptr_Valid(b)
         1
   IDL> *b = 5

I like this because it fits into the programming style
I've developed. For example:

   IF N_Elements(color) EQ 0 THEN color = 5
   IF N_Elements(*b) EQ 0 THEN *b = 5

But again, you must *initialize* this pointer to an
undefined variable in the INIT method, NOT in the __DEFINE
module.

Cheers,

David

-- 
David Fanning, Ph.D.
Fanning Software Consulting
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