Swift - 在 map 中存储值。结构与类中的不同行为

Question

如果将 SomeClass 定义为结构，则此示例 Swift 代码无法编译。编译器说:@value $T6 与 (String, Proto) 不同

但是，如果 SomeClass 是一个类，编译器不会报错。

为什么？

public protocol Proto {    func hello(value:Int)}public struct SomeClass {    var map = [String:Proto]()    public func store (key:String, value:Proto) {        map[key] = value // That does not work if SomeClass is a struct      }}

Answer 1

因为你在store函数中修改了struct的元素，如果你这样做，你必须给它加上“mutating”前缀。所以:

public mutating func store (key:String, value:Proto) {

    map[key] = value // That does not work if SomeClass is a struct

}

Structures and enumerations are value types. By default, the properties of a value type cannot be modified from within its instance methods.

However, if you need to modify the properties of your structure or enumeration within a particular method, you can opt in to mutating behavior for that method.