Scala - mutable (var) method parameter reference

Question

EDIT: I keep getting upvotes here. Just for the record, I no longer think this is important. I haven\'t needed it since I posted it.

I would like to do following in

Answer 1

The JVM does not allow pass-by-reference of pointers to objects (which is how you'd do this in C++), so you can't do exactly what you want.

One option is to return the new value:

def save(srcPath: String, destPath: String): String = {
  val newPath = (if (!destPath.endsWith("/")) destPath+'/' else destPath)
  // do something
  newPath
}

Another is to create a wrapper:

case class Mut[A](var value: A) {}

def save(srcPath: String, destPath: Mut[String]) {
  if (!destPath.value.endsWith("/")) destPath.value += '/'
  // do something
}

which users will then have to use on the way in. (Of course, they'll be tempted to save("/here",Mut("/there")) which will throw away the alterations, but this is always the case with pass-by-reference function arguments.)

---

Edit: what you're proposing is one of the biggest sources of confusion among non-expert programmers. That is, when you modify the argument of a function, are you modifying a local copy (pass-by-value) or the original (pass-by-reference)? If you cannot even modify it it is pretty clear that anything you do is a local copy.

Just do it that way.

val destWithSlash = destPath + (if (!destPath.endsWith("/")) "/" else "")

It's worth the lack of confusion about what is actually going on.