Re: Object Data and pointer assignments

["J.D. Smith" <jdsmith(at)astro.cornell.edu>]

David Fanning wrote:
> 
> 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.
> 

That's a nice idea.  I hadn't thought of doing it that way.  In my method, the
validity of the pointer is what indicates an empty vs. non-empty list.  In your
method, whether the variable pointed to by the pointer is defined provides the
same distinction.  With your method, you save yourself tests like:

	if ptr_valid(ptr) n_elem=0 else n_elem=n_elements(ptr)

(of which I have *many*) in favor of:

	n_elem=n_elements(*ptr)

This is very clean.  To pay for that, though, each time your list (or whatever)
reaches 0 size, you must do a:

ptr_free,ptr
ptr=ptr_new(/ALLOC)

the latter line not being required in my method (a consequence of the
indistinguishability of null pointers and dangling pointers).  I think this
trade is well worth it, though, and I will consider using your method in the
future.  

Thanks for the tip!

JD

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