你能在 Swift 中重载类型转换运算符吗？

Question

我想在 Swift 中实现已知类型之间的自动类型转换。C# 的做法是重载类型转换运算符。如果我希望我的X类型可以与，例如，交叉分配string，我会写：

public class X 
{
     public static implicit operator string(X value)
     {
          return value.ToString();
     }

     public static implicit operator X(string value)
     {
          return new X(value);
     }
}

之后，我可以写这样的东西：

string s = new X();
X myObj = s;

它们会自动转换。这在 Swift 中有任何可能吗？

Answer 1

不，该语言不为自定义类型提供此类功能。Objective-C 集合和 Swift 集合之间存在桥接，但这是内置的且不可自定义。

// Swift array of `String` elements
let swiftArray: [String] = ["Bob", "John"] 

// Obj-C array of `NSString` elements, but element type information 
// is not known to the compiler, so it behaves like an opaque NSArray
let nsArray: NSArray = ["Kate", "Betty"] 

// Obj-C array of an `NSString` and an `NSNumber`, element type
// information is not known to the compiler
let heterogeneousNSArray: NSArray = ["World", 3]

// Casting with `as` is enough since we're going from Swift array to NSArray 
let castedNSArray: NSArray = swiftArray as NSArray

// Force casting with `as!` is required as element type information 
// of Obj-C array can not be known at compile time
let castedSwiftArray: [String] = nsArray as! [String]

// Obj-C arrays can not contain primitive data types and can only 
// contain objects, so we can cast with `as` without requiring a 
// force-cast with `!` if we want to cast to [AnyObject]
let heterogeneousCastedNSArray: [AnyObject] = heterogeneousNSArray as [AnyObject]

有关类型转换的文档可在此处获取。

我认为你可以用初始化器实现你想要做的事情。

extension X {
    init(string: String) {
        self = X(string)
    }        
}

extension String {
    init(x: X) {
        // toString is implemented elsewhere
        self = x.toString
    }        
}

let x = X()
let string = "Bobby"

let xFromString: X = X(string: string)
let stringFromX: String = String(x: x)

与您的问题没有直接关系，但还有一系列以 开头的协议ExpressibleBy...，使您能够执行以下操作： 假设我们要从整数文字初始化字符串。我们可以通过遵守并实施来做到这一点ExpressibleByIntegerLiteral

// Strings can not be initialized directly from integer literals
let s1: String = 3 // Error: Can not convert value of type 'Int' to specified type 'String'

// Conform to `ExpressibleByIntegerLiteral` and implement it
extension String: ExpressibleByIntegerLiteral {
  public init(integerLiteral value: Int) {
    // String has an initializer that takes an Int, we can use that to 
    // create a string
    self = String(value)
  }
}

// No error, s2 is the string "4"
let s2: String = 4 

可以在这里ExpressibleByStringLiteral找到一个很好的用例。