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.