private[this] Vs giving parameters without val/var in scala

Question

I understand that marking a val/var as private[this] inside a class and providing a class parameter without val/var is same.

 class ABC (x:Int){..}

 class ABC {
   private[this] x:Int = 100
 }

In both the cases, x can be accessed by only the constructors and the methods of the same object. Is my understanding correct ?

Thanks!

Dmytro Mitin · Accepted Answer · 2020-11-01T21:14:31.213

Both

import scala.reflect.runtime.universe._

reify{
  class ABC(x: Int)
}.tree

and

reify {
  class ABC(private[this] val x: Int)
}.tree

produce

{
  class ABC extends AnyRef {
    <paramaccessor> private[this] val x: Int = _;
    def <init>(x: Int) = {
      super.<init>();
      ()
    }
  };
  ()
}

And

reify{
  class ABC {
    private[this] val x: Int = 100
  }
}.tree

produces

{
  class ABC extends AnyRef {
    def <init>() = {
      super.<init>();
      ()
    };
    private[this] val x: Int = 100
  };
  ()
}

So the answer to your question

In both the cases, x can be accessed by only the constructors and the methods of the same object.

is positive.

You can check that class ABC(x: Int) does create a (private[this]) field verifying that you can access it with this

class ABC(x: Int) {
  this.x
}

But it's important to emphasize that Scala specification doesn't specify either that x in class ABC(x: Int) is a field or that it's not a field. This is just current behavior of Scalac and Dotty compilers.

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