Swift类型推理和类型检查问题

问题描述

我不是在寻找正确答案，但为什么会发生这种情况。

I'm not looking for an answer like how to do it correctly but why this happens.

这是代码：

func isInt(param: AnyObject?) {
    if let value = param as? Int {
        print(value)
    } else {
        print("Not Int")
    }

    if let value = param {
        if value is Int {
            print(value)
        } else {
            print("Not Int")
        }
    }
}

let a:AnyObject? = 1.2
let b:Float? = 1.2
let c:Double? = 1.2

isInt(a)
isInt(b)
isInt(c)

我理解在第一个如果循环中， param 被转换为 Int 然后打印出 1 。

I understand in the first if loop, the param is casted to Int and then print out 1.

如果循环，如果值为Int 为true，然后打印出 1.2

But why in second if loop, if value is Int is true and then print out 1.2?

推荐答案

在 b $ c> let value = param bridges value 到 NSNumber 类型。 NSNumber ，值为Int 将始终为真。

In your b case, let value = param bridges value to an NSNumber type. For NSNumber, value is Int will always be true.

对于unbridged值：

For unbridged values:

a is Int // always true, AnyObject bridges to NSNumber here
b is Int // false, cast from Float to Int always fails
c is Int // false, cast from Double to Int always fails

b $ b

这个答案假设Foundation已经导入。没有基金会，您的作业将失败。

This answer assumes Foundation has been imported. Without Foundation, your assignments will fail.

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