如何将变量转换为某种运行时类型从Scala中的TypeCast [英] How to cast a variable to certain runtime type got from TypeCast in Scala

问题描述

asInstanceOf [T] 适用于简单情况，其中类型 T 例如

  scala> val x：Any = 5 
 x：Any = 5 
 
 scala> x.asInstanceOf [Int] 
 res50：Int = 5 
 
 scala> val m1：Any = Map（a→1，b→2）其中，a，b，c， 
 
 scala> m.asInstanceOf [Map [String，Int]] 
 res51：Map [String，Int] = Map（a  - > 1,2  - > b）
 
 scala> val m2：Any = Map [any，any]（a - > 1,2  - >b）
 m2：Any = Map（a-> 1,2  - > b） 
 
 scala> m.asInstanceOf [Map [A​​ny，Any]] 
 res52：Map [A​​ny，Any] = Map（a  - > 1，2  - > b）
  / pre> 
 
 
但是当运行时通过 TypeTags  T  c $ c>， asInstanceOf [T] 不起作用。例如：
  scala> val intT = typeTag [Int] .tpe 
 intT：reflect.runtime.universe.Type = Int 
 
 scala> x.asInstanceOf [intT] 
< console>：12：error：not found：type intT 
 x.asInstanceOf [intT] 
  
错误说明 intT 不是类型 。因此 reflect.runtime.universe.Type 不是真实的类型？如何使用 typeTag 信息将值转换为某种类型？
解决方案
  intT 不是类型，而是类型的实例。混乱，肯定。我不确定是否有一些内置的反射API这样做，但你可以很容易地写一个方法，将转换为 A 给定一个元素任何和 TypeTag [A] 。
  def cast [A]（a：Any，tt：TypeTag [A]）：A = a.asInstanceOf [A] 
 
 scala> val intT = typeTag [Int] 
 intT：reflect.runtime.universe.TypeTag [Int] = TypeTag [Int] 
 
 scala> val a：Any = 1 
 a：Any = 1 
 
 scala> cast（a，intT）
 res101：Int = 1 
  
 ：
  implicit class TypeTagCast（a：Any）{
 def fromTypeTag [A]（tt：TypeTag [A] ）：A = a.asInstanceOf [A] 
} 
 
 scala> a.fromTypeTag（intT）
 res106：Int = 1 
  
 
The asInstanceOf[T] works for "simple" case where the type T is given explicitly by code: e.g.
scala> val x: Any = 5
x: Any = 5

scala> x.asInstanceOf[Int]
res50: Int = 5

scala> val m1: Any = Map[String, Int]("a"->1, "b"->2)
m1: Any = Map(a -> 1, b -> 2)

scala> m.asInstanceOf[Map[String, Int]]
res51: Map[String,Int] = Map(a -> 1, 2 -> b)

scala> val m2: Any = Map[Any,Any]("a"->1, 2->"b")
m2: Any = Map(a -> 1, 2 -> b)

scala> m.asInstanceOf[Map[Any, Any]]
res52: Map[Any,Any] = Map(a -> 1, 2 -> b)
But when the type T is retrieved at runtime through TypeTags, asInstanceOf[T] does not work. For example:
scala> val intT = typeTag[Int].tpe
intT: reflect.runtime.universe.Type = Int

scala> x.asInstanceOf[intT]
<console>:12: error: not found: type intT
              x.asInstanceOf[intT]
The error says it clear that intT is not a type. So reflect.runtime.universe.Type is not a real type? How to cast a value to certain type using typeTag information?
解决方案
intT isn't a type, but an instance of Type. Confusing, for sure. I'm not sure if there is something built into the reflection API that does this, but you can easily write a method that will cast to A given an element Any and a TypeTag[A].
def cast[A](a: Any, tt: TypeTag[A]): A = a.asInstanceOf[A]

scala> val intT = typeTag[Int]
intT: reflect.runtime.universe.TypeTag[Int] = TypeTag[Int]

scala> val a: Any = 1
a: Any = 1

scala> cast(a, intT)
res101: Int = 1
Another variant using an implicit class:
implicit class TypeTagCast(a: Any) {
    def fromTypeTag[A](tt: TypeTag[A]): A = a.asInstanceOf[A]
 }

scala> a.fromTypeTag(intT)
res106: Int = 1


                        
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