Swift iOS - 我为什么可以循环遍历一组类对象并进行属性更改但不能更改结构[重复]

Question

如果我遍历 Class 对象的数组，我可以对其上的属性进行更改

class Country {
    var name: String?
    var region: String?

    init(name: String?, region: String?) {
        self.name = name
        self.region = region
    }
}

let canada = Country(name: "Canada", region: "North America")
let mexico = Country(name: "Mexico", region: "North Ameria")
let france = Country(name: "France", region: "Europe")
let korea = Country(name: "Korea", region: "Asia")

var countryArr = [canada, mexico, france, korea]

// this works fine
let transformed = countryArr.map { $0.name = "Random" }

但如果我用Struct 对象尝试这个，我会得到

无法分配给属性：“$0”是不可变的

struct Country {
    var name: String?
    var region: String?
}

var canada = Country(name: "Canada", region: "North America")
var mexico = Country(name: "Mexico", region: "North Ameria")
var france = Country(name: "France", region: "Europe")
var korea = Country(name: "Korea", region: "Asia")

var countryArr = [canada, mexico, france, korea]

// this gets an error
let transformed = countryArr.map { $0.name = "Random" }

Answer 1

这个问题是由于structs 是值类型，所以改变结构的任何属性也会改变结构实例本身，并且map 中的闭包输入参数是不可变的。因此，当您尝试在 map 的闭包中更改属性 $0 时，您正在尝试更改 $0 本身，以防在值类型集合上调用 map。

另一方面，classes 是引用类型，因此改变 class 实例的属性不会改变实例本身。

您的问题的解决方案是在map 中创建结构实例的可变副本，改变其name 属性并返回它。有两种解决方案，如果你的类型有少量属性，调用它的成员初始化器更容易，但是如果你有很多属性并且只想改变一些，复制结构然后修改必要的属性是更好的选择。

let transformed = countryArr.map { Country(name: "Random", region: $0.region) }
let transformed2 = countryArr.map { country->Country in
    var copy = country
    copy.name = "Random"
    return copy
}