Re: set a pointer to const

[Hans Boldt <boldt@xxxxxxxxxx>]

Steve Richter wrote:
> I have C and C++ code that parses command strings and does some specialized
> scanning that I would like to port from visual C++ to ILE. These functions
> accept pointers as arguments, some that are pointers to const data.  In some
> cases prototyping as %string works, but other times simply passing a pointer
> is best.
>
> In general I would like to find out if mix and matching C++ and RPG
> procedures works better than coding entirely in RPG. But the RPG limitation
> of setting a pointer to only non const variables will not work well with C++
> which makes heavy use of const references and pointers.
>
> Basically, if RPG provides the pointer, why restrict it to only non const
> variables?  Esp since const is such a good modular programming practice and
> the solution of qualifying a pointer and DS as const appears to be very
> doable.
If I'm not mistaken, C and C++ also have (annoying) restrictions on
what you can do with const pointers.

In RPG, if you were able to save the address of a CONST parameter,
you could effectively do anything you wanted to the data pointed at
by that pointer. You might muck up data in the passed parameter, or
you might muck up data on the call stack, or you might even change
data in the constant data section of store.

I suppose we could extend the language to allow you to define
pointers as "CONST" in RPG. That would then require some way of
restricting their use. Perhaps variables BASED on a CONST pointer
would have to be considered read-only. (I'll add this to our list of
proposed enhancements.)

In general, there are plenty of good alternatives in RPG to using
pointers (which I sometimes regret adding to the RPG IV language).
As Barbara and I have pointed out, if you're passing a pointer from
C to RPG, you can define the parameter in RPG as either a reference
or a const parameter. Just because you're dealing with pointers in
C/C++ doesn't mean you have to program that way in RPG.

BTW, are you familiar with Barbara's document describing how to call
between C and RPG:  <http://www.opensource400.org/callc.html>

Cheers!  Hans