Map of with object references as keys?

Question

I have an object with stores information about specific instances. For that, i would like to use a Map, but as the keys are not by-reference (they aren\'t, rig

Answer 1

Ah based on comment... You could use a wrapper that overrides equal to have reference semantics.

class EqWrap[T <: AnyRef](val value: T) {
  override def hashCode() = if (value == null) 0 else value.hashCode
  override def equals(a: Any) = a match {
    case ref: EqWrap[_] => ref.value eq value
    case _ => false
  }
}
object EqWrap {
  def apply[T <: AnyRef](t: T) = new EqWrap(t)
}

case class A(i: Int)

val x = A(0)
val y = A(0)

val map = Map[EqWrap[A], Int](EqWrap(x) -> 1)
val xx = map.get(EqWrap(x))
val yy = map.get(EqWrap(y))
//xx: Option[Int] = Some(1)
//yy: Option[Int] = None

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Original answer (based on not understanding the question - I have to leave this so that the comment makes sense...)

Map already has this semantic (unless I don't understand your question).

scala> val x = A(0)
x: A = A(0)

scala> val y = A(0)
y: A = A(0)

scala> x == y
res0: Boolean = true // objects are equal

scala> x.hashCode
res1: Int = -2081655426

scala> y.hashCode
res2: Int = -2081655426 // same hash code

scala> x eq y
res3: Boolean = false // not the same object

scala> val map = Map(x -> 1)
map: scala.collection.immutable.Map[A,Int] = Map(A(0) -> 1)

scala> map(y)
res8: Int = 1 // return the mapping based on hash code and equal semantic