Manifest

最新推荐文章于 2025-12-16 22:04:16 发布

原创最新推荐文章于 2025-12-16 22:04:16 发布 · 151 阅读

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本文探讨了Scala中的类型擦除问题，以及如何通过清单(Manifest)来解决在编译时未知类的情况下创建原生数组的问题。类型关系运算符被视为近似值，因为类型一致性在清单中并未完全体现。示例代码展示了如何使用Manifest进行数组创建，以及如何检查类型间的近似子类型关系。此外，还展示了如何获取方法的返回类型信息。

清单[T]是类型T的晦涩描述符。它的支持用途是作为类实例提供对类型擦除的访问，如果类在编译时未知，这是创建原生数组所必需的。
A Manifest[T] is an opaque descriptor for type T. Its supported use is to give access to the erasure of the type as a Class instance, as is necessary for the creation of native Arrays if the class is not known at compile time.

类型关系运算符<:<和=:=应仅被视为近似值，因为类型一致性的许多方面在清单中尚未得到充分体现。
The type-relation operators <:< and =:= should be considered approximations only, as there are numerous aspects of type conformance which are not yet adequately represented in manifests.


scala> def arr[T] = new Array[T](0)   
<console>:11: error: cannot find class tag for element type T
       def arr[T] = new Array[T](0)
                    ^

scala> def arr[T](implicit m: Manifest[T]) = new Array[T](0)
arr: [T](implicit m: Manifest[T])Array[T]

scala> def arr[T: Manifest] = new Array[T](0)               
arr: [T](implicit evidence$1: Manifest[T])Array[T]

scala> def isApproxSubType[T: Manifest, U: Manifest] = manifest[T] <:< manifest[U]
warning: there was one deprecation warning; re-run with -deprecation for details
isApproxSubType: [T, U](implicit evidence$1: Manifest[T], implicit evidence$2: Manifest[U])Boolean

scala> isApproxSubType[List[String], List[AnyRef]]
res5: Boolean = true

scala> isApproxSubType[List[String], List[Int]] 
res6: Boolean = false

scala> def methods[T: Manifest] = manifest[T].runtimeClass.getMethods
 
methods: [T](implicit evidence$1: Manifest[T])Array[java.lang.reflect.Method]

scala> def retType[T: Manifest](name: String) =
   methods[T] find (_.getName == name) map (_.getGenericReturnType)
     | retType: [T](name: String)(implicit evidence$1: Manifest[T])Option[java.lang.reflect.Type]