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操作符。如果我希望我的X类型可以与string交叉分配，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);
     }
}

之后我可以写下这样的东西：

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

它们会自动转换。这在Swift中有可能吗？

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，但它已经融入并且无法自定义。

// 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]

提供类型转换的文档here。

我认为你可以用初始化器来实现你想做的事。

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，但也有一系列以ExpressibleBy...开头的协议，使您可以执行以下操作：假设我们想要从整数文字初始化字符串。我们可以通过遵守和实施ExpressibleByIntegerLiteral

来实现这一目标

// 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

可以找到ExpressibleByStringLiteral的一个很好的用例here。

你能在Swift中重载类型转换操作符吗？

1 个答案: