在 Kotlin 中，“::class.simpleName”是做什么的？

Question

val var1: Any = "Carmelo Anthony"

印象中::class.simpleNamereturns对象的变量类型
当我执行以下操作时：

val var1Type = var1::class.simpleName
print(var1Type)

我得到 String 而不是 Any

但是当我这样做的时候

val var2: String = var1

我得到一个Type mismatch: inferred type is Any but String was expected

Answer 1

在 Kotlin 中，::class 运算符有两种形式：
- TypeName::class - which returns a KClass object for the static type TypeName.
- variableName::class - variableName 的 which returns a KClass object corresponding to the runtime type，并且不是 variableName 的静态类型。（Kotlin 在其文档中称其为“绑定类型”）。
在您的例子中，var1 的运行时类型为 String，但静态类型为 Any。
- 所以 var1::class returns String 的 KClass，而不是 Any。
但是 Kotlin 的类型系统，像大多数静态类型语言一样，不允许隐式缩小转换（即给定一个类型为 String 的变量 var2，你不能 assign-to var2 来自另一个变量 (var3) statically-typed 为 Any，因为 var3 可能具有与String，例如 InputStream 对象。
- ...即使也可以证明（通过遵循程序 by-hand）Any 类型的值将始终是 String.
- 然而，幸运的是，Kotlin 的 type-checker 是现代的 and its "Smart cast" feature follows the scope of type-narrowing 当使用 is 运算符时，这很整洁（TypeScript 也有，我认为任何其他语言都没有确实如此）。
  - 在您不能使用 Smart-casts 或者可以用其他方式证明向下转型是安全的情况下，请使用 as 运算符来执行一个 unsafe cast。像这样：var2: String = var1 as String。
    - （有点令人困惑，其他语言使用 as 作为 safe 转换的运算符，argh）。

In context:

fun main() {

    val var1: Any = "Carmelo Anthony"
    val var1Type = var1::class.simpleName
    println("var1's type: " + var1Type) // <-- This will print the *runtime type* of `var1` (String), not its static type (which is `Any`, *not* `String`).

    /*
    val var2: String = var1 // <-- Fails beause `var1` is `Any`, and `Any` is "wider" than `String`, and narrowing conversions always considered unsafe in languages like Kotlin, Java, etc.
    */
    val var2Unsafe: String  = var1 as  String; // <-- Doing this is unsafe because it will throw if `var1` is not a String.
    val var2Safe  : String? = var1 as? String; // <-- Doing this is safe because it `var2Safe` will be null if `var1` is not a String.
    
    println(var2Unsafe)
    println(var2Safe)
}

如果您熟悉其他语言，那么这里有一个不完整的 table 等效操作及其语法：

	Kotlin	Java	JavaScript	C#	C++
Get static type	`TypeName::class`	`TypeName.class`	`ConstructorName`	`typeof(TypeName)`	`typeid(TypeName)`
Get runtime type	`variableName::class`	`variableName.getClass()`	`typeof variableName` (intrinsics) `variableName.constructor` (objects)	`variableName.GetType()`	`typeid(variableName)`
Get type from name (string)	`Class.forName( typeName ).kotlin`	`Class.forName( typeName )`	`eval( typeName )` (never do this)
Statically-defined runtime type check	`variableName is TypeName`	`variableName instanceof TypeName`	`typeof variableName === 'typeName'` (intrinsics) or `variableName instanceof ConstructorName` (objects)	`variableName is TypeName`
Runtime dynamic type check	`otherKClass.isInstance( variableName )` or `otherKType.isSubtypeOf()`	`otherClass.isAssignableFrom( variableName.getClass() )`		`otherType.IsAssignableFrom( variableName.GetType() )`
Unsafe narrowing (aka downcast)	`val n: NarrowType = widerVar as NarrowType;`	`NarrowType n = (NarrowType)widerVar;`	`variableName as TypeName` (TypeScript only)	`NarrowType n = (NarrowType)widerVar;`
Safe narrowing (downcast or `null`)	`val n: NarrowType? = widerVar as? NarrowType;`			`NarrowType n? = widerVar as NarrowType;`	`dynamic_cast<NarrowType>( widerVar )`
Conditional narrowing in scope	`variableName is TypeName`		`func(x: unknown): x is TypeName` guard functions (TypeScript only)	`widerVar is TypeName n`

在 Kotlin 中，“::class.simpleName”是做什么的？

In Kotlin, what does "::class.simpleName" do?

type-inference

variable-assignment

type-mismatch

assignment-operator

kotlin