Scala case class private constructor isn't private

Question

Today I encountered a strange problem with case class constructors. I wanted to make a constructor private and it seems that it isn't problem. So I've tried it in one of my projects and it works. But in another project I can invoke private constructor and it compiles. I thought it is something with my ide, so I made a standalone class and compile it with scalac. And it compiles. Here is the code:

package com.test

object Main {

  def main(args: Array[String]) {
    val bar = Bar("12345")
//    bar.doStuff()
    println(bar)
  }
}

case class Bar private(foo: String){
  private def doStuff():Unit = println("stuff")
}

The funny thing is that if I uncomment bar.doStuff() it won't compile. So I assume private works in this case, but somehow doesn't work for constructor. What am I doing wrong? Scalac is 2.11.8

Answer 1

The notation val bar = Bar("12345") is shorthand for val bar = Bar.apply("12345"), in other words, it calls the apply method of the (automatically generated) companion object of the case class.

The companion object has access to the private constructor, so that's why it works.

(Why do you want to make the constructor of a case class private? That doesn't sound like a good idea).

Answer 2

If you want Bar("abc") to not compile, here's a simple workaround.

Quickly tested in 2.12.3. Doesn't work in 2.11.8

case class Bar private (value: String)
object Bar {
  private def apply(v: String) = new Bar(v) // makes the method private
  def create(value: String): Bar = ... // your own implementation
}

val bar = Bar("abc") // compile error
val bar = new Bar("abc") // compile error
val bar = Bar.create("abc") // ok