Swift协议继承&通用功能 [英] Swift Protocol Inheritance & Generic functions

问题描述

考虑以下游乐场：

import Foundation

protocol StringInitable {
    init( string:String )
}

class A : StringInitable {
    var stored:String

    required init ( string:String ) {
        stored = string
    }
}

class B : A /*, StringInitable */ {
    var another_stored:String

    required init ( string:String ) {
        another_stored = "B-store"

        super.init(string: string)
    }
}

func maker<T:StringInitable>(string:String) -> T {
    return T(string: string)
}

let instanceA = A(string: "test-maker-A")
let instanceB = B(string: "test-maker-B")

let makerA:A = maker("test-maker-A")
let makerB:B = maker("test-maker-B")

let typeInstanceA = _stdlib_getTypeName(instanceA)
let typeMakerA = _stdlib_getTypeName(makerA)

let typeInstanceB = _stdlib_getTypeName(instanceB)
let typeMakerB = _stdlib_getTypeName(makerB)

从结果中，编译器似乎已推断出正确的类型但未能调用正确的初始化器。为什么我必须在B类中显式实现StringInitable（通过删除B类定义中的注释来测试）以使泛型函数maker调用正确的初始值设定项？

From the results the compiler seems to have inferred the correct types but has failed to call the correct initializers. How come I have to explicitly implement StringInitable in the B-class (test by removing the comment in the B class definition) to have the generic function "maker" call the correct initializer?

推荐答案

这有点像编译器错误，原因很简单： makerB 是 B的变量 type，但它被分配了一个 A 的实例。这应该是不可能的，事实上如果你试图打印，并且更一般地来说是访问 makerB的 another_stored 属性变量，引发运行时异常，我不会期待别的。

That smells like a compiler bug for one simple reason: makerB is a variable of B type, but it is assigned an instance of A. This should not be possible, and in fact if you try to print, and more generally to access to, the another_stored property of the makerB variable, a runtime exception is raised, and I wouldn't expecting nothing else.

这是因为如果 B 是 A 的子类， A 的实例无法分配给<$ c $的变量c> B 类型（反之亦然）。

That's because if B is a subclass of A, an instance of A cannot be assigned to a variable of B type (whereas the opposite is possible).

将变量 A 类型分配给变量 B 类型是可能的，但仅在这些条件下：

Assigning a variable of A type to a variable of B type is possible though, but only under these conditions:

• 来自 A 到 B 已完成（否则编译器应该出错）


• A引用的实例变量实际上是 B 的实例（否则应该引发运行时异常）

• an explicit downcast from A to B is done (the compiler should error otherwise)
• the instance referenced by the A variable is actually an instance of B (a runtime exception should be raised otherwise)

请注意，编译器不仅无法调用正确的初始化程序 - 它调用了另一个类的初始化程序

Note that the compiler didn't just fail to call the correct initializer - it called the initializer of another class

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