如何为扩展中的协议实现 CustomStringConvertible？

Question

我想在协议扩展中为我的协议实现一个可打印的功能，然后任何符合协议的结构都将根据协议的字段进行打印。但我收到以下错误，似乎 swift 不允许这样做。我应该扩展String 并为其添加init(_:Info) {...} 吗？

protocol Info {
    var name: String { get }
    var serial: Int { get }
}
struct Person : Info {
    var name = ""
    var serial = 0
}

extension Info : CustomStringConvertible {
                 ^~~~~~~~~~~~~~~~~~~~~~~
                 error: extension of protocol 'Info' cannot have an inheritance clause

    var description: String {
        return "\(name) + \(serial)"
    }
}
func print(info: Info) {
    print(info)
}

Answer 1

您应该在定义中继承协议，而不是在扩展中。

protocol Info: CustomStringConvertible {
    var name: String { get }
    var serial: Int { get }
}
struct Person : Info {
    var name = ""
    var serial = 0
}

extension Info  {
    var description: String {
        return "\(name) + \(serial)"
    }
}
func print(info: Info) {
    print(info)
}

Answer 2

您需要使您的协议符合CustomStringConvertible。然后在您的extension 或Info 中，您可以提供默认方法。如下。

protocol Info: CustomStringConvertible {
    var name: String { get }
    var serial: Int { get }
}

struct Person: Info {
    var name = ""
    var serial = 0
}

extension Info {
    var description: String {
        return "\(name) + \(serial)"
    }
}

func print(info: Info) {
    print(info)
}

这是因为一致性需要一个约束，否则编译器认为它是一个继承

另一种方法是执行以下操作：

protocol Info {
    var name: String { get }
    var serial: Int { get }
}

struct Person: Info, CustomStringConvertible { // notice that Person now conforms to CustomStringConvertible
    var name = ""
    var serial = 0
}

// the extension only applies to objects that themselves conform to CustomStringConvertible
extension Info where Self: CustomStringConvertible  {
    var description: String {
        return "\(name) + \(serial)"
    }
}