Defining object for case class

Question

As far as I got in the case of case class companion object is generated by the compiler automatically.

case class Clas(i: Int)

But in my case I want to add some fatory method apply(s: String): Clas for convenience. So defined object myself as:

object Clas {
     def apply(s: String) = //create Clas
}

How does it work? Why does methods from compiler-generated object are still available?

Answer 1

The compiler merges your methods with the synthetic ones in the companion object:

scala> :past
// Entering paste mode (ctrl-D to finish)

case class Clas(i: Int)
object Clas { def apply(s: String): Clas = null }

// Exiting paste mode, now interpreting.

defined class Clas
defined object Clas

scala> Clas.apply _
<console>:13: error: ambiguous reference to overloaded definition,
both method apply in object Clas of type (i: Int)Clas
and  method apply in object Clas of type (s: String)Clas
match expected type ?
       Clas.apply _
            ^