We started with VALUE. Now it's time to talk about CONST and compare the two keywords: the pros and cons and when you should use each. Even if you always use these keywords, read on. You might be surprised.

Let's start with a quick recap of the previous TechTip. In RPG, parameters passed between programs are, by default, passed by reference. In other words, what is passed when program A calls program B are the references to the locations (or addresses) in memory of the parameters used, not the contents of the parameters. However, when you call a procedure or a function, you're allowed to choose how the parameters should be passed.

If the keyword VALUE is used in a parameter, then what will happen is that a copy of the parameter's contents will be created in memory and passed to the called program, thus passing its value instead of its reference. In practical terms, this means that whatever changes are made to the parameter in the called procedure won't be reflected in the calling program/procedure. That's the VALUE keyword in a nutshell.

CONST, however, works differently. When you use the CONST keyword, the parameter is passed by reference, but (and this is the main advantage) you won't be allowed to change its contents intentionally. The compiler returns a compilation error if your code tries to change the parameter's contents. This may sound a bit confusing, so let's analyze an example:

P MyProc         B                   EXPORT

D MyProc         PI             N          

D Parm_A                      10A   VALUE  

D Parm_B                       4P 0 CONST  

* (...)                                    

C                   Eval     Parm_A = 'A'  

C                   Eval     Parm_B = 42

If you tried to compile this piece of code, the compilation would fail because PARM_B is defined in the procedure interface with the CONST keyword. This means that you cannot change its value directly at compilation time.

That's the beauty of CONST: it helps your code speak for itself! If you define a parameter with CONST, what you're really saying is "the contents of this parameter are to remain the same (i.e., constant) throughout the execution of this procedure." Some programmers argue that you should define all your input parameters with CONST: it clearly states that they are input parameters and therefore cannot be changed. I partially agree with this approach, because you cannot intentionally change a variable defined with CONST. You can actually change the contents at runtime, in debug mode. It's any interesting experience; try it! Anyway, note the word intentionally. It's possible to change the contents unintentionally when there's a buffer overflow (caused by poorly managed pointers or bad prototypes declaration) and part of the parameter's content gets overwritten. These situations are extremely difficult to identify and solve.

VALUE Versus CONST

Ok, let's imagine that I change the procedure shown above and remove the line that changes PARM_B. It's now possible to compile and call MyProc. Let's also imagine that I have another program that calls this procedure, like so:

C                   CallP     MyProc(Parm_A : Parm_B)

No matter what happens in MyProc, the original (in other words, the calling program's) Parm_A variable will never get changed (a bit like "What happens in MyProc stays in MyProc"). Because Parm_A is defined with VALUE in MyProc, what will happen "behind the scenes" is that the system will copy the contents of Parm_A to a new variable and pass that variable's memory address (its pointer) to MyProc. This is important to know, because if Parm_A was a big variable—say, a 16Mb array—instead of a simple 10-character string, it would take a bit to duplicate it before MyProc could be called. And this will happen every single time the procedure is called.

However, this doesn't happen with Parm_B: its contents are passed by reference and, as I said before, cannot be explicitly changed. (Actually, I said intentionally, but let's go with explicitly now, just for the sake of argument.) Parm_B can be changed in yet another way: if you happen to have a field that has the same name as the parameter. You can argue that this might not happen with a parameter called Parm_B, but just for a moment let's imagine that someone would actually create a field called Parm_B in one of the files used by MyProc. Whenever that new record of that file is read, Parm_B's contents would be implicitly changed. The big problem here is that the changed contents would be passed back to the calling program when the procedure ended. Unlike VALUE, CONST only prevents compile-time changes to the parameter's contents, not run-time changes.

Just a quick note regarding variable/parameter naming: you should never use field names as parameter names, and you should also use prefixes for files used in a procedure or program! I'll go over a bunch of ground rules later in this series, with a TechTip exclusively dedicated to good naming conventions and practices.

So What Keyword Should I Use for My Input Parameters?

As you can see, both keywords have pros and cons. You need to be careful when defining your procedure's parameters, taking into account the what the parameters' sizes are, what you're going to do with them and, if you know in advance, how often will the procedure get called.

VALUE is nice because you're 100% sure (unless something really, really weird happens) that the original variable is not changed. However, it's not a good idea to use this keyword in intensively used procedures or big parameters because the system will have to copy the parameter's contents to a new variable every single time the procedure is called.

CONST is also nice because, since there's no content copying, the procedure gets called immediately regardless of the size of the parameters; only the pointers are passed. It's faster and also "cleaner" in the sense that you're not allowed to intentionally change the parameter's contents. Just use proper naming and be careful with the prototype definitions! You will be able to avoid those nasty buffer overflow and I/O nightmares I mentioned previously.

As usual, your feedback is most welcome! I try to incorporate as much reader feedback as possible into my articles (Sébastien, Jared, and Chris, this is for you this time) because I humbly believe that there's always room for improvement and that everyone can learn from each other every day.

The next TechTip will continue to discuss parameters, covering the OPTIONS keyword and all that is associated with it. Until next time!