RE: Is const really that much better than value?

Larry,

There's been some discussion about this in the past. In particular look for posts from Barbara Morris
on the subject.

Here's one:
http://archive.midrange.com/rpg400-l/200705/msg00155.html

"Neither CONST nor VALUE is always better than the other. But if you _were_ restricted to one or the
other, CONST would be a better choice.

For a few data types (pointer, float, integer), VALUE is a bit more efficient. For other data types,
CONST is usually more efficient; the larger the parameter, the better CONST is. For large parameters,
CONST is more efficient even if you have to copy the parameter to a temporary within the procedure to
have something you can modify; this is especially true of VARYING parameters."

Whenever possible, if I'm passing large strings, or to be more precise, if my procedure accepts large
strings, I make them CONST VARYING.


HTH,
Charles

-----Original Message-----
From: rpg400-l-bounces@xxxxxxxxxxxx
[mailto:rpg400-l-bounces@xxxxxxxxxxxx] On Behalf Of Larry Ducie
Sent: Tuesday, October 30, 2007 8:17 AM
To: rpg400-l@xxxxxxxxxxxx
Subject: Is const really that much better than value?

Hi Chaps,

Quick question - when passing a variable value to a
subprocedure, and the passed variable does not have the same
length/precision as that defined on the prototype, what is
the real difference between passing it by constant reference
or by value? Why choose one over the other? For passing long
strings by constant reference and options(*varsize) is not
used then doesn't the OS simply copy the variable to a
temporary field anyway if the variable length does not match
the prototype definition? Isn't this the same as passing by
value in all real respects?

I mean, when parameters are passed to a subprocedure the OS
passes an array of pointers together with operational
descriptors doesn't it? for passing by value the pointers
will point to temporary variables containing copies of the
original data. These are cast/padded/truncated accordingly.
When passing by constant reference doesn't the OS also do
this if the original variable definition does not exactly
match that on the prototype? This is why we can pass zoned
fields even though the prototype defines a packed parm, etc.

I guess I'm basicaly asking - in this scenario is there any
real performance reason for passing a long string by constant
reference over passing it by value? Do we only get a
performance gain when we have an exact match and a temporary
field is not necessary when passing by constant reference? (I
must stress thet I'm talking about scenarios when
options(*varsize) and/or opdesc are NOT used).

I'm very interested to know the mechanics behind this so I
can make the right design choices. If I have it all wrong
then I'd rather know now. :-)

Cheers

Larry Ducie

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