| Re: Pass by value and performance [message #46787 is a reply to message #46719] |
Fri, 16 December 2005 02:24   |
Antonio Santiago
Messages: 201
Registered: February 2004
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Senior Member |
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JD Smith wrote:
> On Thu, 15 Dec 2005 07:56:50 +0100, Antonio Santiago wrote:
>
>
>> Kenneth P. Bowman wrote:
>>
>>> Perhaps someone can clarify this for me.
>>>
>>> I was doing this
>>>
>>> data = {values : FLTARR(...), $
>>> other : other stuff ...}
>>>
>>> Then pass "data" to a procedure and do this
>>>
>>> result = INTERPOLATE(data.values, x, y, z)
>>>
>
>
>
>> I like to understand pointers in IDL in this way:
>>
>> 1.- 'a' is a conventional variable managed by IDL and its "garbage
>> collector".
>
>
> Sadly, IDL doesn't have garbage collection. It would be nice if it
> did, but until then, it's up to you to free all of your heap variables
> at the correct time (which is great when you know when that is).
>
>
>> 2.- '*a' is a HEAP variable, where 'a' stores a reference to it. Also, the
>> content of the variable 'a' is stored in the heap memory.
>>
>> Then 'a' is a reference for a "normal" variable that stores a reference,
>> and '*a' is a reference to a HEAP variable that stores a 5.
>
>
> I'd just say both a and *a are variables. One ordinary (local in
> scope), the other heap (global in scope).
>
>
Sorry, but unfortunately yesterday a bad boy was sitting in my chair and
writte the above misspelling words. Alse the bad boy is a bad englighs
witter :( (like me ;) ).
>> junk, *a --> The content of the HEAP memory variable is pased by value.
>
>
> This isn't correct. De-referenced pointer variables (aka "heap"
> variables) are passed by reference, just like regular variables (which
> they are, really). E.g. in Ken's original example:
>
> result = INTERPOLATE(*data.array, x, y, z) ; by reference
>
> would indeed pass the pointer heap variable by reference and not by
> value. As such it would be much faster (for large arrays) than
> INTERPOLATE(data.array,x,y,z), which would require copying the full
> array to a local variable, and would be equivalent to a simple
> INTERPOLATE(array,x,y,z).
>
> As pointed out in the pointer tutorial
> (http://www.dfanning.com/misc_tips/pointers.html), there is no
> difference between pointer heap variables and ordinary variables,
> except in how you access them. Of course, that also means that a
> structure member (or array element, etc.) of a dereferenced pointer
> variable is (just like a member of an ordinary variable), still passed
> by value:
>
> result = INTERPOLATE((*data).array, x, y, z) ; by value
>
> Here `data' is a pointer to a structure with member "array", which is
> passed here by value.
>
> This equivalence also means that standard IDL variable tricks, like
> re-assigning the memory contents of one variable to another without
> copying, work just fine for pointer heap variables (and in between
> plain old variables and pointer heap variables).
>
> JD
>
OK, I think I understand IDL pointer. Maybe my problem was to find the
similities between C pointers and IDL pointers. That is, when I saw '*a'
I read the C style: "the content where 'a' points to".
Following this I have:
a = 10
b = PTR_NEW(40)
c = PTR_NEW( BYTARR(100) )
Conventional Mem. | HEAP memory
(Managed by IDL but | (Jungle where you are
without a Garbage Collector??? ) | responsible to free)
------------------------------------------------------------ ----
a = 10 |
b ---------------------------|--> 40
c ---------------------------|--> [0,1,2,.......,99]
Then if I call a function with:
call_to_procedure, *c
In bad C style I think I am passing the content of 'c', that is a BTYARR
of 100 (BAD ???)
In IDL is passed a reference to the content, that is like if I writte:
d = BYTARR(100)
call_to_procedure, d
Is this right??
Thanks a lot.
PD: I think I must talk seriously with the bad boy of my office :)
--
-----------------------------------------------------
Antonio Santiago P�rez
( email: santiago<<at>>grahi.upc.edu )
( www: http://www.grahi.upc.edu/santiago )
( www: http://asantiago.blogsite.org )
-----------------------------------------------------
GRAHI - Grup de Recerca Aplicada en Hidrometeorologia
Universitat Polit�cnica de Catalunya
-----------------------------------------------------
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