How to instantiate an instance of type represented by type parameter in Scala

Question

example:

import scala.actors._  
import Actor._  

class BalanceActor[T <: Actor] extends Actor {  
  val workers: Int = 10  

  private lazy val actors = new Array[T](workers)  

  override def start() = {  
    for (i <- 0 to (workers - 1)) {  
      // error below: classtype required but T found  
      actors(i) = new T  
      actors(i).start  
    }  
    super.start()  
  }  
  // error below:  method mailboxSize cannot be accessed in T
  def workerMailboxSizes: List[Int] = (actors map (_.mailboxSize)).toList  
.  
.  
.

Note the second error shows that it knows the actor items are "T"s, but not that the "T" is a subclass of actor, as constrained in the class generic definition.

How can this code be corrected to work (using Scala 2.8)?

Answer 1

EDIT - apologies, I only just noticed your first error. There is no way of instantiating a T at runtime because the type information is lost when your program is compiled (via type erasure)

You will have to pass in some factory to achieve the construction:

class BalanceActor[T <: Actor](val fac: () => T) extends Actor {
  val workers: Int = 10

  private lazy val actors = new Array[T](workers)

  override def start() = {
    for (i <- 0 to (workers - 1)) {
      actors(i) = fac() //use the factory method to instantiate a T
      actors(i).start
    }
    super.start()
  }
}

This might be used with some actor CalcActor as follows:

val ba = new BalanceActor[CalcActor]( { () => new CalcActor } )
ba.start

As an aside: you can use until instead of to:

val size = 10
0 until size //is equivalent to:
0 to (size -1)

Answer 2

Use Manifest:

class Foo[A](a: A)(implicit m: scala.reflect.Manifest[A]) {
  def create: A = m.erasure.newInstance.asInstanceOf[A]
}

class Bar

var bar1 = new Bar       // prints "bar1: Bar = Bar@321ea24" in console
val foo = new Foo[Bar](bar1)
val bar2 = foo.create    // prints "bar2: Bar = Bar@6ef7cbcc" in console
bar2.isInstanceOf[Bar]   // prints "Boolean = true" in console

BTW, Manifest is undocumented in 2.7.X so use it with care. The same code works in 2.8.0 nightly as well.

Answer 3

You can't, as mentioned already, instantiate T because of erasure. At run-time, there is no T. This is not like C++'s templates, where the substitution happens are compile-time, and multiple classes are actually compiled, for each variation in actual use.

The manifest solution is interesting, but assumes there is a constructor for T that does not require parameters. You can't assume that.

As for the second problem, the method mailboxSize is protected, so you can't call it on another object. Update: this is true only of Scala 2.8.