Kotlin实体类型参数不会智能投射 [英] Kotlin reified type parameter doesn't smart cast

问题描述

我正在尝试设置未初始化的值，并试图让下面的工作。这主要是对泛化泛型的强大（和限制）的好奇。

我试图为数据类的可选参数提供默认值。 b
$ b

  inline fun< reified T> uninitialized（）：T = when（T :: class）{
 Long :: class  - > -1L //类型不匹配。要求：T找到：长
 String :: class  - >  // 类型不匹配。 Required：T Found：String 
 //等等... 
 else  - >抛出UnsupportedOperationException（没有为+ T :: class定义未初始化的值）
 
 
数据类Thing（
 var id：Long =未初始化的（），
 var name：String = uninitialized（）//等等... 
）
  

当包含时，是类型子句时，Kotlin具有智能铸造。在这个例子中，智能铸造不会踢，所以这不会编译。

任何想法可以完成类似的事情？

is 来检查其类型或将其与<$>进行比较之后，将智能转换应用于特定对象C $ C>空。在您的示例中，您没有检查类型的具体对象，也没有适用于智能演员的内容。

但是，您可以将手动演员应用到 T ，这将按预期工作。下面是你的示例函数的一个工作版本，已更新，以处理Kotlin的反射库的特性，该库将在1.1中修复：

  inline有趣的东西T：任何> uninitialized（）：T = when（T :: class.java）{
 Long :: class.javaPrimitiveType，Long :: class.javaObjectType  - > -1L as T 
 String :: class.java  - > as T 
 //等等... 
 else  - >抛出UnsupportedOperationException（没有为+ T :: class定义未初始化的值）
 
 
数据类Thing（
 var id：Long =未初始化的（），
 var name：String = uninitialized（）//等等... 
）
 
 fun main（args：Array< String>）{
 val t = Thing（） 
 println（t.id）
} 
  

I was experimenting with setting uninitialized values and was trying to get the following to work. This is mostly a curiosity in the power (and limitations) of reified generics.

I was attempting to provide default values for optional parameters of data classes.

inline fun <reified T> uninitialized(): T = when (T::class) {
  Long::class -> -1L // Type mismatch. Required: T  Found: Long
  String::class -> "" // Type mismatch. Required: T  Found: String
  // and so on...
  else -> throw UnsupportedOperationException("No uninitialized value defined for " + T::class)
}

data class Thing(
    var id: Long = uninitialized(),
    var name: String = uninitialized() // and so on...
)

When when includes is Type clauses, Kotlin has smart casting. In this example, smart casting isn't kicking in so this will not compile.

Any ideas to accomplish something similar?

解决方案

A smart cast is applied to a specific object after you use is to check its type or compare it with null. In your example, there is no specific object for which you check the type, and nothing to apply the smart cast to.

However, you can apply manual casts to T, which will work as expected. Here's a working version of your sample function, updated to handle the peculiarities of Kotlin's reflection library which will be fixed in 1.1:

inline fun <reified T : Any> uninitialized(): T = when (T::class.java) {
  Long::class.javaPrimitiveType, Long::class.javaObjectType -> -1L as T      
  String::class.java -> "" as T
  // and so on...
  else -> throw UnsupportedOperationException("No uninitialized value defined for " + T::class)
}

data class Thing(
    var id: Long = uninitialized(),
    var name: String = uninitialized() // and so on...
)

fun main(args: Array<String>) {
    val t = Thing()
    println(t.id)
}

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