Private and protected constructor in Scala

Question

I've been curious about the impact of not having an explicit primary constructor in Scala, just the contents of the class body.

In particular I suspect that the private or protected constructor pattern, i.e. controlling construction through the companion object or another class or object's methods might not have an obvious implementation.

Am I wrong? if so how is it done please?

Answer 1

You can declare the default constructor as private/protected by inserting the appropriate keyword between the class name and the paramater list, like this:

class Foo private () { 
  /* class body goes here... */
}

Answer 2

Aleksander's answer is correct, but Programming in Scala offers an additional alternative:

trait Foo {
 // interface
}

object Foo {
  def apply(...) = // public constructor

  private class FooImpl(...) extends Foo { ... } // real class
}