Scala case class private constructor but public apply method

Question

If I have the following case class with a private constructor and I can not access the apply-method in the companion object.

case class Meter private (m: Int)

val m = Meter(10) // constructor Meter in class Meter cannot be accessed...

Is there a way to use a case class with a private constructor but keep the generated apply-method in the companion public?

I am aware that there is no difference (in my example) between the two options:

val m1 = new Meter(10)
val m2 = Meter(10)

but I want to forbid the first option.

-- edit --

Surprisingly the following works (but is not really what i want):

val x = Meter
val m3 = x(10) // m3  : Meter = Meter(10)

What version of Scala are you using? I just tried it in my 2.10.0 REPL and `val m2 = Meter(10)` does not give any error] — Luigi Plinge, Nov 17 '13 at 16:47
Seems like it's the line `case class Meter private (m: Int)` that causes the error, when declared as top level object (http://scalafiddle.net/console/eb6fdc36b281b7d5eabf33396c2683a2) but it works when declared within another object or the REPL (http://scalafiddle.net/console/cdc0d6e63aa8e41c89689f54970bb35f) — Luigi Plinge, Nov 17 '13 at 19:47

score 52 · Answer 1 · edited May 23 '17 at 11:54

52

Here's the technique to have a private constructor and a public apply method.

trait Meter {
  def m: Int
}

object Meter {   
  def apply(m: Int): Meter = { MeterImpl(m) }
  private case class MeterImpl(m: Int) extends Meter { println(m) }
}

object Application extends App {
  val m1 = new Meter(10) // Forbidden
  val m2 = Meter(10)
}

Background information private-and-protected-constructor-in-scala

edited May 23 '17 at 11:54

Community

1
1

answered Nov 17 '13 at 13:10

@senia, no it doesn't. Here's the compile error for new Meter(10){} ; trait Meter is a trait; does not take constructor arguments – Nov 17 '13 at 13:44
1

I missed that. You should define `Meter` as `trait Meter { def m: Int }` to allow access to `m`. You could also make it `sealed`. – senia Nov 17 '13 at 13:46
2

I think this is a good workaround because you get the asked behavior and some of the benefits from case classes (e.g. implemented equals). But it's kind of heavy compared to the single case class. – Erik Nov 17 '13 at 13:51
2

In this case wouldn't `println(Meter(1))` result in `> MeterImpl(1)`? Not that this doesn't answer the question, but I'd like a solution where that doesn't happen. – Ritwik Bose Jun 05 '15 at 01:44

score 5 · Answer 2 · answered Jun 22 '18 at 17:33

It seems the requested behavior (private constructor but public .apply) may be the way Scala 2.12 implements these.

I came to this from the opposing angle - would like a private case class constructor also block the .apply method. Reasons here: https://github.com/akauppi/case-class-gym

Interesting, how use cases differ.

Yuriy · Answer 3 · 2017-07-17T09:33:42.343

It is possible with some implicit tricks:

// first case 
case class Meter[T] private (m: T)(implicit ev: T =:= Int)
object Meter { 
  def apply(m: Int) = new Meter(m + 5) 
}

created some another constructor (and apply method signature) but guaranty that parameter can be only Int.

And after you have case class with case class features (with pattern matching, hashcode & equals) exclude default constructor:

scala> val m = Meter(10)
m: Metter[Int] = Meter(15)

scala> val m = new Meter(10)
<console>:9: error: constructor Meter in class Meter cannot be accessed in object $iw
       val m = new Meter(10)

OR with type tagging (naive implementation):

trait Private
case class Meter private (m: Integer with Private)
object Meter {
  def apply(m: Int) = new Meter((m + 5).asInstanceOf[Integer with Private])
}

It works as expected:

val x = new Meter(10)
<console>:11: error: constructor Meter in class Meter cannot be accessed in object $iw
              new Meter(10)
              ^

val x = Meter(10)
x: Meter = Meter(15)

Some possible issues with primitive types and type tags described here

Julian Pieles · Answer 4 · 2023-04-18T11:00:18.170

1

The solutions above are rather complicated. It is just:

case class Meter private (m: Int)
object Meter { 
 def apply(m: Int): Meter = new Meter(m)
}

Test

new Meter(10) // fails to compile
Meter(10) // compiles just fine

edited Apr 18 '23 at 11:00

answered Apr 18 '23 at 10:54

Julian Pieles

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Scala case class private constructor but public apply method

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