Polymorphic instantiation in Scala using TypeTag and ClassTag

reflection, scala, scala-2.10

Solution

`TypeTag` is not yet a replacement for `Manifest` because it's a part of experimental and unstable Scala reflection. You definitely shouldn't use it for production as of now.

For the use case you showed, where only runtime class is needed (not full type information with generics etc.), Scala 2.10 introduced `ClassTag`, which you can use like this:

def newInstance[T: ClassTag] =
  implicitly[ClassTag[T]].runtimeClass.newInstance.asInstanceOf[T]

or:

def newInstance[T](implicit ct: ClassTag[T]) =
  ct.runtimeClass.newInstance.asInstanceOf[T]

Anyway, `Manifest` isn't deprecated yet, so I guess you can still use it.

EDIT:

Using `TypeTag` to achieve the same:

import scala.reflect.runtime.universe._

def newInstance[T: TypeTag] = {
  val clazz = typeTag[T].mirror.runtimeClass(typeOf[T])
  clazz.newInstance.asInstanceOf[T]
}

The above solution still uses some Java reflection. If we want to be puristic and use only Scala reflection, this is the solution:

def newInstance[T: TypeTag]: T = {
  val tpe = typeOf[T]

  def fail = throw new IllegalArgumentException(s"Cannot instantiate $tpe")

  val noArgConstructor = tpe.member(nme.CONSTRUCTOR) match {
    case symbol: TermSymbol =>
      symbol.alternatives.collectFirst {
        case constr: MethodSymbol if constr.paramss == Nil || constr.paramss == List(Nil) => constr
      } getOrElse fail

    case NoSymbol => fail
  }
  val classMirror = typeTag[T].mirror.reflectClass(tpe.typeSymbol.asClass)
  classMirror.reflectConstructor(noArgConstructor).apply().asInstanceOf[T]
}

Problem

In Scala 2.9 one could implement polymorphic instantiation as ``` def newInstance[T](implicit m: Manifest[T]) = m.erasure.newInstance.asInstanceOf[T] ``` but as of 2.10 `Manifest` is being replaced with `TypeTag`, and it is not clear to me how to achieve something similar with `TypeTag`. I would prefer if the TypeTag version preserved all available type information. I know that the above only works for traits/classes that do not require constructor args, and ven then it does not always work, but it works well enough for what I need. If I can do better the new reflection APIs that would be great.

Original source