How to pass a class method as a parameter in Scala

Question

Lets say I have a class C with some methods

def class C {
  def f1():Int = ...
  def f2():Int = ...
}

Now I'd like a method that takes two instances of C, as well as a method of C, but I don't know what the types of f1, f2 are, nor how to invoke them. I'm thinking it would look something like

def cmp(first:C, second:C, t:() => Int): Boolean = {
  first.t < second.t
}

This complains that t is not a method of C. Surely there must be a way to express this.

Answer 1

def cmp(first:C, second:C, t: C => Int): Boolean = {
  t(first) < t(second)
}

Then...

val c1 = new C
val c2 = new C
cmp(c1, c2, _.f1())
cmp(c1, c2, _.f2())

This is using anonymous functions. The last two lines are equivalent to:

cmp(c1, c2, {c: C => c.f1()})
cmp(c1, c2, {c: C => c.f2()})

You can't pass a reference to a method per se unless you use some kind of reflection.

Answer 2

You can just pass the method references instead:

object DoOperation extends App {

  class C(val x: Int) {
      def f1():Int = x-1
      def f2():Int = x+1
   }


  def cmp(first: () => Int, second: () => Int): Boolean = {
    first() < second()
  }

  override def main(args: Array[String]) {
      println(cmp(new C(1).f1,new C(0).f2)) //prints "true"
      println(cmp(new C(1).f2,new C(1).f1)) //prints "false"
  }

}

The methods will be closed over their corresponding object instances, so the net effect is the equivalent to what you appear to want to accomplish.