Do I need to import members of a singleton object into its companion class in Scala?

Question

The Good Book states that:

A class and its companion object can access each other’s private members.

Perhaps naively, I took this as meaning that a class didn't need to explicitly import the members from its companion object. I.e., the following would work:

object Foo {
  def bar = 4
 }

class Foo {
 def foo = bar
}

Well, the reason you're reading this is that it doesn't. So do I really need to declare something like this:

class Foo {
  import Foo._

  def foo = bar
}

score 17 · Accepted Answer · answered Aug 19 '10 at 22:17

17

Yes, you do, just as you state. There's access, and there's scope -- what companion class/objects have is access, not scope.

It's like declaring something public vs private -- it doesn't bring those members into everyone's scope, just give them access to it.

answered Aug 19 '10 at 22:17

Daniel C. Sobral

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2

see http://stackoverflow.com/q/23704769/86485 for some further discussion on _why_ this is the case – Seth Tisue Jun 17 '15 at 16:28

score 4 · Answer 2 · answered Aug 20 '10 at 07:56

4

"Can access private members" means that the following works:

object Foo {
  private def bar = 4
}

class Foo {
  def foo = Foo.bar
}

answered Aug 20 '10 at 07:56

Alexey Romanov

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If desired, you can restrict access with `private[this]` def bar = 4`. – retronym Aug 20 '10 at 09:51

score 3 · Answer 3 · answered Aug 19 '10 at 22:18

3

Yes (and I want my 15 points for that!)

But to expand, their scopes do not overlap, so the import is necessary.

answered Aug 19 '10 at 22:18

Randall Schulz

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Sorry, you were 53 seconds late :-) – lindelof Aug 19 '10 at 23:23
1

I know, dammit! That Daniel character is going to be the death of me. – Randall Schulz Aug 20 '10 at 00:34

Do I need to import members of a singleton object into its companion class in Scala?

3 Answers3