如何在Scala中实例化由type参数表示的类型的实例 [英] How to instantiate an instance of type represented by type parameter in Scala

问题描述

例子：

$ $ p $ import scala.actors._
import Actor._

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

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

覆盖def start（）= {
for（i < - 0 to（workers - 1））{
//以下错误：需要classtype，但T找到
actors（i） = new T
actors（i）.start
}
super.start（）
}
//错误如下：方法mailboxSize无法在T $ b中访问$ b def workerMailboxSizes：List [Int] =（actors map（_.mailboxSize））。toList
。
。
。


注意第二个错误表明它知道演员项目是T，但不是T是actor的一个子类，正如类泛型定义中的约束一样。

这段代码如何纠正才能工作（使用Scala 2.8）？

解决方案

编辑 - 道歉，我只注意到你的第一个错误。没有办法在运行时实例化 T ，因为在编译程序时（通过类型擦除），类型信息会丢失。

您必须通过一些工厂来实现构建：

  class BalanceActor [T<：Actor]（val fac：（）=> T）扩展Actor {
 val workers：Int = 10 
 
 private lazy val actors = new Array [T] （i <= 0到（workers-1））{
 actors（i）= fac（）//（$（））{
覆盖def start（）= {
 //使用工厂方法实例化T 
 actors（i）.start 
} 
 super.start（）
} 
} 
  

这可能与一些演员 CalcActor 一起使用，如下所示：

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

另外：您可以使用，直到而不是到：

  val size = 10 
 0直到大小//相当于：
 0到（大小-1）
  

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)?

解决方案

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)

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