Clever Geek Handbook

How to refer to an outer object from an inner class in Scala

Consider this code (which is a type of safe units):

abstract class UnitsZone {
   type ConcreteUnit <: AbstractUnit

   abstract class AbstractUnit(val qty: Int) {
     SOME_ABSTRACT_MEMBERS
     def +(that: ConcreteUnit): ConcreteUnit = POINT_OF_INTEREST.apply(this.qty + that.qty)
     def apply(param: Int) = SOME_IMPLEMENTATION
   }

   def apply(qty: Int): ConcreteUnit
}

object Imperial extends UnitsZone {
  type ConcreteUnit = Pound

  class Pound(override val qty: Int) extends AbstractUnit(qty) {
    CONCRETE_MEMBERS_HERE
  }

  def apply(qty: Int) = new Pound(qty)
}

To do all this, I need to call a method of apply

an external object regarding inheritance (marked as POINT_OF_INTEREST in the above code). With this in mind, I dare to ask a few questions:

  • Is there a way to do this?
  • If there are many, what are the pros and cons for each?
  • If you think this is all wrong, what is your correct way to implement such functionality?
+3


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2 answers


Use your own link:

abstract class UnitsZone {
  outer =>
  type ConcreteUnit <: AbstractUnit
  ...
  abstract class AbstractUnit(val qty: Int) {
    def +(that: ConcreteUnit): ConcreteUnit = outer.apply(this.qty + that.qty)
    ...
  }
}


See chapter 17. Self-referenced SO Scala tutorial for more information on what else this construct can do.

+7


source


Just like Java,

class Foo {
  val foot = 0
  class Bar {
    val bart = Foo.this.foot
  }
}


or Scala,

class Foo { self =>
  val foot = 0
  class Bar {
    val bart = self.foot
  }
}
+3


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