Accompanying object for trait in Scala

Question

I try to write general apply method to serve as a factory method for child classes. What I came up with is following:

trait A 

object A {
    def apply[T <: A](someParam: String): T {
        new T()
    }
}

class B private extends A 
class C private extends A

So one can create new B or C instance:

def main(args: Array[String]): Unit = {
    A[B]("test")
}

Is it acceptable that trait has an accompanying object?Is there a better way to implement this?

Answer 1

You could:

  trait A

  object A {
    def apply[T <: A ](someParam: String) (implicit ev: ClassTag[T]): T ={
      ev.runtimeClass.newInstance().asInstanceOf[T]
    }
  }

But its not very useful with varying constructor's for each sub class

  sealed trait A

  object A {
    def apply[T <: A](someParam: String)(implicit ev: T): T = {
      ev
    }
  }

  class B(n: Int) extends A    
  class C(n: String) extends A

  object B {
    implicit def defaultCons: B = new B(1)
  }

  object C {
    implicit def defaultCons: C = new C("hi")
  }

Or you could also go via combination of ClassTag approach with pattern match based on the class instead of using reflection for instantiation.