不能对类型的不可变值使用变异成员 [英] Cannot use mutating member on immutable value of type

问题描述

我有以下结构:

public protocol SuperModel {
    // empty protocol
}
struct ModelOne: SuperModel {
    struct SubModelOne {
        var someVar: Double
        var othervar: Double?
    }
    var sub: SubModelOne?
    mutating func setSub(sub: SubModelOne) {          
        self.sub = sub
    }
}

在我的课堂上，我想像这样使用这个结构:

In my class, I want to use this struct like that:

final class SomeClass: SuperClass {
    var data: SuperModel
    init() {
        self.data = ModelOne()
    }
    func someFunc() {
        (self.data as! ModelOne).setSub(ModelOne.SubModelOne(someVar: 2, otherVar: 1))
    }
}

我收到以下错误:无法在ModelOne"类型的不可变值上使用可变成员.为什么会这样，我该如何解决?

I get following error: Cannot use mutating member on immutable value of type 'ModelOne'. Why is that so and how can I fix this?

推荐答案

当您将类型转换应用于值类型(例如结构体)时，如果成功，您将收到请求值的不可变副本:>

When you apply type casting to value types (such structs), if succeed, you receive immutable copy of requested value:

(self.data as! ModelOne) // this is copy of data

唯一的方法(据我所知)如何改变需要转换的值 - 重新分配值(正如@Sahil Beri 指出的，你需要声明变量):

The only way (as known to me) how you can mutate values that need to be casted - reassign value (as @Sahil Beri pointed you need declare variable):

func someFunc() {
    if var data = data as? ModelOne {
        data.setSub(ModelOne.SubModelOne(someVar: 2, otherVar: 1))
        self.data = data // you can do this since ModelOne conforms to SuperModel
    }
}

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