需要类类型,但发现T

Question

我出于某种原因无法创建一个actor(这是我的类层次结构的简单版本):

abstract class Class1[T <: Class2[_]: ClassTag] extends Actor {
   //....
   val res = List(1, 2, 3) map { x => context actorOf Props(new T(x)) } // error
}

abstract class Class2[U <: Class3 : ClassTag](a: Int) extends Actor { ... }
abstract class Class3(b: Int) extends Actor

但是有一个错误说 class type required but T found.

Answer 1

您无法new T(x)使用type参数调用T.可能没有这样的构造函数T:

class WithoutSuchConstructor extends Class2[Class3](1)

您应该指定T明确创建的方法:

abstract class Class1[T <: Class2[_]: ClassTag] extends Actor {
  //....
  def createT(i: Int): T
  val res = List(1, 2, 3) map { x => context actorOf Props(createT(x)) }
}

或者:

abstract class Class1[T <: Class2[_]: ClassTag](createT: Int => T) extends Actor {
  //....
  val res = List(1, 2, 3) map { x => context actorOf Props(createT(x)) }
}

Answer 2

我使用的一种方法涉及创建一个'instantiator'特征,该特征可用于创建存在隐式实例化器的类型实例:

trait Instantiator[+A] {
  def apply(): A
}

object Instantiator {
  def apply[A](create: => A): Instantiator[A] = new Instantiator[A] {
    def apply(): A = create
  }
}

class Foo() { ... }

object Foo {
  implicit val instantiator: Instantiator[Foo] = Instantiator { new Foo() }
}

// def someMethod[A]()(implicit instantiator: Instantiator[A]): A = {
def someMethod[A : Instantiator](): A = {
  val a = implicitly[Instantiator[A]].apply()
  ...
}

someMethod[Foo]()