swift – 如何在引擎盖下进行元组比较工作

let p1 = (name: "John",age:12)
let p2 = (color: "Red",size:12)


if p1 == p2 {
    print("equal")
}else {
    print("not equal")
}

func givePerson() -> (name: String,age: Int)? {
    return ("alex",2)
}

func extract() {
    var p3 : (Name: String,age: Int)

    if let person = giveName() as? (Name: String,age: Int) {
        p3 = person
        print(p3)
    }else {
        print("p3 not defined")
    }

}

extract() // p3 not defined

标准库 defines tuple comparison operators包含2个元素元组,最多包含6个元素元组.它通过使用元编程工具 GYB(Generate Your Boilerplate)来实现这一点,该工具允许嵌入式Python代码从模板生成文件.

生成的元组比较运算符最终看起来像：

public func == <A,B>(lhs: (A,B),rhs: (A,B)) -> Bool

public func == <A,B,C>(lhs: (A,C),C)) -> Bool

public func == <A,C,D>(lhs: (A,D),D)) -> Bool

// ...

所以用你的代码：

let p1 = (name: "John",age: 12)
let p2 = (color: "Red",size: 12)

if p1 == p2 {
    print("equal")
} else {
    print("not equal")
}

编译器将调用两元素元组比较运算符.您会注意到,定义的元组比较运算符都没有为其参数使用元组标签.之所以(name：String,age：Int)和(color：String,size：Int)都可以传递给(A,B)的原因是编译器实现了一个可以去除其标签元组的隐式转换.因此,两个参数都剥离了它们的标签,并且都作为(String,Int)传递.

这就是使以下合法的原因：

let p1 = (name: "John",age: 12)
let p2: (String,Int) = p1 // Legal.

事情变得更加奇怪,编译器还有一个隐式转换,可以向元组添加任意标签：

let p1 = ("John",12)
let p2: (foo: String,bar: Int) = p1 // Legal.

但是禁止直接重命名标签：

// Indirect renaming ?
let p1 = (name: "John",Int) = p1
let p3: (foo: String,bar: Int) = p2

// Put together ?
let p4: (foo: String,bar: Int) = p1 as (String,Int)

// Direct renaming ?
let p4 = (name: "John",age: 12)
let p5: (foo: String,bar: Int) = p4

这就是你所看到的行为？投.与上面的示例一样,首先强制转换为未标记的表单：

func givePerson() -> (name: String,age: Int)? {
  return ("alex",2)
}

func extract() {
  var p3: (Name: String,age: Int)
  if let person = givePerson() as (String,Int)? as? (Name: String,age: Int) {
    p3 = person
    print(p3)
  } else {
    print("p3 not defined")
  }

}

extract() // (Name: "alex",age: 2)