Scala protected object

Question

In Scala, if I create an object and companion class, identifiers declared with the protected modifier can be accessed from the class if the object is imported:

object Foo {
  protected val X = 42
}
class Foo {
  import Foo._
  def getX(): Int = X
}

However, the protected identifier cannot be accessed from a subclass of the class Foo:

class Bar extends Foo {
  import Foo._
  def getX(): Int = X * 2
}

I get a compile-time error in Bar.

Other then (implied) public, is there any access modifier I can place on X so that it can be accessed from subclasses of its companion, but not from other classes, including other classes in the same package?

Answer 1

That's because only the class Foo is companion to the object Foo.

Here, the difference between private and protected meaningless, since the object Foo is a singleton, which means there isn't any other object that has the same class as object Foo (Foo.type).

Access restriction in Scala is package-based, so the short answer is no. You could make a forwarder on the base class, though, unless you need it to be available without an instance.

In your place, however, I'd go back to the design board.

Answer 2

In such cases, I would suggest using a package private modifier, like below:

object Foo {
  private[your_package] val X = 42
}

The value will still be visible to everybody else in the package.