Swift的pow()函数不会接受Doubles作为参数 [英] Swift's pow() function won't accept Doubles as arguments

问题描述

我创建了这个infix operator ^^来代替使用pow函数:

I created this infix operator ^^ as a substitute to using the pow function:

infix operator ^^ { associativity left precedence 155 }
func ^^ <T: IntegerLiteralConvertible>(left: T, right: T) -> T {
    return pow(left as Double, right as Double)
}

我将IntegerLiteralConvertible协议用作泛型left和right的类型约束，因为据我了解，此图表显示，它基本上包括所有数字类型.

I used the IntegerLiteralConvertible protocol as a type constraint for the generics left and right, because from my understanding this diagramm shows, that it basically includes all number types.

为了使用pow函数，我必须将left和right向下转换为Double，这是我使用as运算符完成的.这不是最安全的方法，但是那不是重点.

In order to use the pow function I have to downcast left and right to Double though, which I did using the as operator. It's not the safest approach, but that's besides the point.

在实现功能时以此方式 swift告诉我:

When implementing the function this way swift tells me:

<stdin>:4:12: error: cannot invoke 'pow' with an argument list of type '(Double, Double)'
return pow(left as Double, right as Double)
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

据我所知，pow将两个Double作为参数，那么为什么会抱怨呢?

Now as far as I know pow takes two Doubles as parameters, so why is it complaining about this?

推荐答案

为什么要抱怨呢?

why is it complaining about this?

因为pow返回Double.并且Double与T不同.该错误消息具有误导性，但是它的意思是无法找到接受(Double, Double)并返回T类型的pow"

Because pow returns Double. And Double is not identical to T. The error message is misleading, but it means "Cannot find pow that accepts (Double, Double) and returns T type"

我认为您在Swift中误解了"cast".在Swift中，as不会转换数字类型.

I think you are misunderstanding "cast" in Swift. In Swift as does not convert numeric types.

let intVal:Int = 12
let doubleVal:Double = intVal as Double
//                            ^ [!] error: 'Int' is not convertible to 'Double'

如果操作数的类型在编译时不可预测，则无效的转换会导致运行时错误:

And If the type of operand is not predictable at compile time, invalid casting causes runtime error:

func foo<T: IntegerLiteralConvertible>(x: T)  {
    x as Double // <-- Execution was interrupted
}
foo(12 as Int)

相反，我们必须明确地转换"它们.请参阅文档:数值类型转换

Instead, we must explicitly "convert" them. see the document: Numeric Type Conversion

let intVal:Int = 12
let doubleVal:Double = Double(intVal)

这仅因Double具有init(_ v: Int)初始化程序而起作用.以下代码无法编译:

This works only because Double has init(_ v: Int) initializer. The following code does not compile:

func foo<T: IntegerLiteralConvertible>(x: T)  {
    Double(x)
//  ^~~~~~~~~ [!] error: cannot invoke 'init' with an argument of type 'T'
}

因为Double没有init<T:IntegerLiteralConvertible>(_ val:T)初始化程序.

因此，如果要使用pow()，则必须将T转换为Double作为参数，并将Double转换为T以返回值.而且没有简单的解决方案.

So, if you want to use pow(), you must convert T to Double for arguments, and convert Double to T for returning value. And there is no simple solution for that.

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