Basic Scala OOP question - pass by reference?

Question

I'm a little stumped at how silly this problem is and have a serious mindblank, but I thought I might ask anyway.

I have an object Foo, with several fields. I want a method that can change any of its fields depending on which one is passed in as a parameter. Like so:

class Foo {
    var x = 0
    var y = 0
}

class Bar {
    def changeFooField(field : Int) = {
        field = 1        
    }
}

Can I not use it like so?:

changeFooField(foo.x)

If not, how do I accomplish this?

Answer 1

A common idiom is to pass an explicit setter method which takes care of changing the value.

class Foo {
  var x = 0
  var y = 0
}

class Bar {
  def changeFooField(setter: (Int) => Unit) = {
    setter(1)
  }
}

val foo = new Foo
val bar = new Bar
bar.changeFooField(foo.x = _) // or equivalent: bar.changeFooField((i: Int) => foo.x = i)

assert(foo.x == 1)

(foo.x = _) is a simple ad-hoc closure which allows write access to foo.x. There is no need in adding this setter API to Foo itself.

So, as it turns out, the setter method (foo.x= – or rather foo.x_=) is passed as an argument exactly as any other method is passed. You’ll have to remember that val and var in Scala don’t actually specify variables but create the methods which are then used to access the real (hidden) variables. (val only creates a getter method, whereas var of course creates both getter and setter).