generics - Kotlin中的简单泛型函数失败

Question

这是Kotlin中的一个简单的泛型函数:

fun <T> twice(x: T) : T { return 2 * x }

尝试构建它(在项目或REPL中)会导致以下错误:

error: none of the following functions can be called with the arguments supplied: 
public final operator fun times(other: Byte): Int defined in kotlin.Int
public final operator fun times(other: Double): Double defined in kotlin.Int
public final operator fun times(other: Float): Float defined in kotlin.Int
public final operator fun times(other: Int): Int defined in kotlin.Int
public final operator fun times(other: Long): Long defined in kotlin.Int
public final operator fun times(other: Short): Int defined in kotlin.Int
fun <T> twice(x: T) : T { return 2 * x }
                                   ^

如果将return语句操作数切换为x * 2，则错误消息将变为:

error: unresolved reference. None of the following candidates is applicable because of receiver type mismatch: 
@InlineOnly public inline operator fun BigDecimal.times(other: BigDecimal): BigDecimal defined in kotlin
@InlineOnly public inline operator fun BigInteger.times(other: BigInteger): BigInteger defined in kotlin
fun <T> twice(x: T) : T { return x * 2 }
                                   ^

我在这里想念什么？

Answer 1

由于T可以是任何东西，因此编译器无法找到匹配的times运算符。如您在错误消息中所看到的，对于Int，有多种选择

public final operator fun times(other: Byte): Int defined in kotlin.Int
public final operator fun times(other: Double): Double defined in kotlin.Int
public final operator fun times(other: Float): Float defined in kotlin.Int
public final operator fun times(other: Int): Int defined in kotlin.Int
public final operator fun times(other: Long): Long defined in kotlin.Int
public final operator fun times(other: Short): Int defined in kotlin.Int

但不幸的是，没有通用的 times函数，例如 Number。恐怕在这种情况下，您必须为要处理的每种类型创建重载，即 Double， Int等。