RE: Procedure Parameter Passing - Processing Implications of Const

Thanks for the feedback Tony & Lim.

Kurt Anderson
Application Developer
Highsmith Inc

-----Original Message-----
From: rpg400-l-bounces@xxxxxxxxxxxx
[mailto:rpg400-l-bounces@xxxxxxxxxxxx] On Behalf Of Tony Carolla
Sent: Wednesday, July 27, 2005 1:39 PM
To: RPG programming on the AS400 / iSeries
Subject: Re: Procedure Parameter Passing - Processing Implications of
Const

On 7/27/05, Kurt Anderson <kjanderson@xxxxxxxxxxxxx> wrote:
> 
> When I define a procedure parameter as Const:
> 1. The caller passes a pointer to the procedure, which the procedure 
> then interprets.
> 2. If I pass an array 32555A Dim(9000), it's no different than if I 
> pass a 3i 0 field (processing-wise)?
> 
> I question this because if I can pass an expression, there is no 
> pointer to that expression... so what is it doing?


Correct. When you pass a parm by reference (the default), only a pointer
is passed. Therefore, if you pass a huge 32555A dim(9000), or a 3I 0,
only the pointer is passed. If you specify to pass the parms by VALUE,
then you are passing the entire contents byte-by-byte. 

The CONST keyword on a parameter spec has a few desirable effects. First
of all, if you specify a 5S 0 parameter with the CONST keyword, then you
can use any numeric field to pass to it in your code. If you pass a 10P
0 to the procedure, the system will create a temporary 5S 0 field, place
the contents of the 10P 0 in the temporary field, and pass the parm
(pointer) to the procedure. If you specify an expression, the same thing
happens. The expression is resolved, moved to a temporary field, and a
pointer to the temporary field is passed to the procedure. This is a
good reason to always check for value limits when passing a potentially
larger variable to a subprocedure parm with CONST specified. The
compiler will not balk, but at runtime, if an overflow occurs, you will
get an exception. 

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