如何投射通用T符合协议 [英] How to cast generic T to conform to protocol

问题描述

我需要在类中转换泛型以符合协议。因为容器类必须被序列化，所以我不能使用约束。所以我怎么能在这种情况下将T转换为ZNumeric，当我已经知道（我可以检查，因为你看到），它符合协议？

  //：Playground  - 名词：人们可以玩的地方
 
导入UIKit 
 
协议ZNumeric {
 
} 
 
扩展名Double：ZNumeric {
 
} 
 
 class GenericClass< T> {
 
} 
 
 class RestrictedGenericClass< T：ZNumeric> {
 
} 
 
 class容器{
需要init？< T>（type：T.Type）{
 let a = GenericClass< T> ;（）
 print（a）
 
如果T.self为ZNumeric.Type {
 print（is numeric）
 // let b = RestrictedGenericClass< T>（）//不会明显工作
 //打印（b）
} 
} 
} 
 
让cDouble =容器（类型： Double.self）//如果T.self是ZNumeric.Type是true 
 let cString = Container（type：String.self）//如果T.self是ZNumeric.Type是false 
  
 
 
  class Container {
 required init？< T>（type ：T.Type）{
 let a = GenericClass< T>（）
 print（a）
} 
 
需要init？< T：ZNumeric>（类型：T.Type）{
 let a = GenericClass< T>（）
 print（a）
 
 print（is numeric）
 let b = RestrictedGenericClass< T>（）
 print（b）
} 
} 
  
更具体的初始化程序将在编译时选择。
 
I need to cast the generic type in a class to conform to a protocol. I can not use constraints since the container class has to be serialized. So how could i cast in this case the T to ZNumeric when i allready know (i can check as you see) that it conforms to the protocol?
//: Playground - noun: a place where people can play

import UIKit

protocol ZNumeric {

}

extension Double: ZNumeric {

}

class GenericClass<T> {

}

class RestrictedGenericClass<T:ZNumeric> {

}

class Container {
    required init?<T>(type: T.Type) {
        let a = GenericClass<T>()
        print(a)

        if T.self is ZNumeric.Type {
            print("is numeric")
            //let b = RestrictedGenericClass<T>() // Will not work obviously
            //print(b)
        }
    }
}

let cDouble = Container(type: Double.self) // if T.self is ZNumeric.Type is true
let cString = Container(type: String.self) // if T.self is ZNumeric.Type is false

解决方案
Because generics have to be known at compile-time, you cannot just check at run-time for conformance (I'm pretty sure). I think this is more like what you want:
class Container {
    required init?<T>(type: T.Type) {
        let a = GenericClass<T>()
        print(a)
    }

    required init?<T : ZNumeric>(type: T.Type) {
        let a = GenericClass<T>()
        print(a)

        print("is numeric")
        let b = RestrictedGenericClass<T>()
        print(b)
    }
}
The more specific initialiser will be chosen at compile-time.

                        
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