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无法将“String”类型的返回表达式转换为返回类型“String”

protocols swift

Jacob M. Barnard · 技术社区 · 7 年前

这是代码中有效的部分。。。

struct Dog<String> {
    typealias Element = String
    var dogName: String
}

extension Dog: ExpressibleByArrayLiteral {
    init(arrayLiteral: Element...) {
        let startIndex = arrayLiteral.startIndex
        let stringName = arrayLiteral[startIndex]
        self.init(dogName: stringName)
    } //end of init(arrayLiteral: Element...) for extension Domino
}


var d: Dog = ["Roofus", "Tony", "Rover"]
print("The dog's name is \(d.dogName)")
var d2: Dog = [1]
print("The dog's name is \(d2.dogName)")
var d3: Dog = [true]
print("The dog's name is \(d3.dogName)")

CustomStringConvertible 扩展,就像这样。。。

extension Dog: CustomStringConvertible {
    var description: String {
        get {
            return "testing with a constant string"
        }
    }
}

error: type 'Dog<String>' does not conform to protocol 'CustomStringConvertible'
extension Dog: CustomStringConvertible {
^
Swift.CustomStringConvertible:67:16: note: protocol requires property 'description' with type 'String'; do you want to add a stub?
    public var description: String { get }
               ^
.../temp.swift:23:9: note: candidate has non-matching type 'String'
    var description: String {
        ^
.../temp.swift:25:20: error: cannot convert return expression of type 'String' to return type 'String'
            return "testing with a constant string"
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                                    as! String
[Finished in 0.1s with exit code 1]

请解释一下为什么编译器会做出以下回应: cannot convert return expression of type 'String' to return type 'String'

extension Dog: CustomStringConvertible {
    var description: String {
        get {
            let s: String = "testing with a constant string"
            return s
        }
    }
}

…只是改变了反馈,即。。。

.../temp.swift:25:20: error: cannot convert return expression of type 'String' 
to return type 'String'
            return "testing with a constant string"
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                                    as! String

到

.../temp.swift:25:20: error: cannot convert value of type 'String' 
to specified type 'String'
            return "testing with a constant string"
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                                    as! String

编辑

1 回复 | 直到 7 年前

Jacob M. Barnard 7 年前

查看的评论 NobodyNada 和 Alexander ,我想出来了。

Dog typealias 那个 Element 被分配到…我想我必须这样做才能 ExpressibleByArrayLiteral 狗 类型

事实上,一个人甚至不需要泛型来制作 可表达的二进制文字 协议)。

struct Dog {
    typealias Element = String
    var dogName: String
}

extension Dog: ExpressibleByArrayLiteral {
    init(arrayLiteral: Element...) {
        let startIndex = arrayLiteral.startIndex
        let stringName = arrayLiteral[startIndex]
        self.init(dogName: stringName)
    } //end of init(arrayLiteral: Element...) for extension Dog
}


var d: Dog = ["Roofus", "Tony", "Rover"]
print("The dog's name is \(d.dogName)")
var d2: Dog = ["1"]
print("The dog's name is \(d2.dogName)")
var d3: Dog = ["true"]
print("The dog's name is \(d3.dogName)")

extension Dog: CustomStringConvertible {
    var description: String {
        let s: String = "testing with a constant string"
        return s
    }
}

var d4: Dog = ["x"]
print("\(d4)")

输出:

The dog's name is Roofus
The dog's name is 1
The dog's name is true
testing with a constant string
[Finished in 0.1s]