RE: Value vs. Const

I prefer to use CONST unless I know that the called procedure needs to pass
the value along to another proc/pgm where it could be modified; then I use
VALUE. If the called proc needs to modify the value, that VALUE is you're
only choice.  Of course if you don't want the benefits of VALUE and CONST,
(casting data to the desired form) then you don't use either.


Bob Cozzi
Cozzi Consulting
www.rpgiv.com


-----Original Message-----
From: rpg400-l-bounces@xxxxxxxxxxxx [mailto:rpg400-l-bounces@xxxxxxxxxxxx]
On Behalf Of Joe Pluta
Sent: Thursday, August 07, 2003 3:04 PM
To: 'RPG programming on the AS400 / iSeries'
Subject: Value vs. Const

Okay, I know the issue was discussed not too long ago, but I'm trying to
make sure I understand.  I'm going full bore with procedures right now
and I'm trying to decide which to use, VALUE or CONST.

It's my understanding that either keyword prevents a parameter from
being modified by the called procedure.  I seem to recall reading that
with CONST, not even the local copy can be modified, while with VALUE,
the local copy can be modified, but won't be passed back.

Thus, a side effect of either keyword is that the compiler can do
auto-conversion, converting literals and constants among other things to
the correct size (though not performing alpha/numeric conversions).
These size conversions are nice in testing, and also for using named
constants.

My question is whether there is a difference in the actual mechanism of
the passing of the parameter.  I have to believe that with VALUE, a copy
is made of the original variable and this copy is passed.  With CONST, I
suppose it's possible that a pointer to the original data is passed, and
it's up to the compiler to prevent modification.  That would be one
reason why the compiler flags attempts to modify local parameters
declared as CONST, but not those declared as VALUE.

The reason I ask is that I'm trying to set up some programming
standards.  Should I always pass parameters as CONST (except when they
are truly going to be modified)?  Or are there cases (let's say the data
is only 2 bytes long) where it might be more efficient to use VALUE,
because only the actual data is being passed on the stack, not a
pointer.

Joe

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