Re: Structure field concatenation

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

Amara Graps (Amara.Graps@mpi-hd.removethis.mpg.de) writes:

> I appreciate your answer, but then I am back to the same error
> I inquired about a couple of weeks ago, i.e.:
> 
> If I do this:
> thisstruc = {orbit:'',freq:ptr_new()}
> instead of this:
> thisstruc = {orbit:'',freq:ptr_new(/allocate_heap)}
> 
> I get this error when I start to create an array of structures 
> and fill it:
> 
> periodcube = replicate(thisstruc,1)
> periodcube(0).orbit = 'G2'
> *periodcube(0).freq=DINDGEN(100)  ;first pointer array is len 100
> 
> % Unable to dereference NULL pointer: <POINTER  (<NullPointer>)>.

Exactly. A NULL pointer is an *invalid* pointer. Hence,
it cannot be dereferenced. Only valid pointers can be
dereferenced. A pointer to an undefined variable *is*
a valid pointer and can be dereferenced, but if you 
replicate the same pointer in a bunch of structures
all the pointers are to the same variable. It is
indeed an oscillating universe. :-)

The solution, I think, is to check to see (if you have
no other way of knowing in your code) if you have
a valid pointer reference before trying to fill the
field with data. Something like this:

   thisstruc = {orbit:'',freq:ptr_new()}
   structs = Replicate(thisStruc, 10)
   IF Ptr_Valid(structs[5].freq) THEN $
      *structs[5].freq = FLTARR(100) ELSE $
      structs[5].freg = Ptr_New(FLATARR(100))

Cheers,

David
-- 
David Fanning, Ph.D.
Fanning Software Consulting
Phone: 970-221-0438 E-Mail: davidf@dfanning.com
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