17-Swift高级运算符

1.溢出运算符

• Swift的算数运算符出现溢出时会抛出运行时错误
• Swift有溢出运算符(&+、&-、&*)，用来支持溢出运算

var min = UInt8.min
print(min &- 1) // 255, Int8.max

var max = UInt8.max
print(max &+ 1) // 0, Int8.min
print(max &* 2) // 254, 等价于 max &+ max

2.运算符重载

• 类、结构体、枚举可以为现有的运算符提供自定义的实现，这个操作叫做:运算符重载

struct Point {
    var x: Int, y: Int
    static func + (p1: Point, p2: Point) -> Point {
        Point(x: p1.x + p2.x, y: p1.y + p2.y)
    }

    static func - (p1: Point, p2: Point) -> Point {
        Point(x: p1.x - p2.x, y: p1.y - p2.y)
    }

    static prefix func - (p: Point) -> Point {
        Point(x: -p.x, y: -p.y)
    }

    static func += (p1: inout Point, p2: Point) {
        p1 = p1 + p2
    }

    static prefix func ++ (p: inout Point) -> Point {
        p += Point(x: 1, y: 1)
        return p
    }

    static postfix func ++ (p: inout Point) -> Point {
        let tmp = p
        p += Point(x: 1, y: 1)
        return tmp
    }
    
    static func == (p1: Point, p2: Point) -> Bool {
        (p1.x == p2.x) && (p1.y == p2.y)
    }
}

//func + (p1: Point, p2: Point) -> Point {
//    Point(x: p1.x + p2.x, y: p1.y + p2.y)
//}

let p = Point(x: 10, y: 20) + Point(x: 11, y: 22)
print(p) // Point(x: 21, y: 42)

3.Equatable

• 要想得知2个实例是否等价，一般做法是遵守Equatable协议，重载==运算符
• 与此同时，等价于重载了 ！= 运算符
• Swift为以下类型提供默认的Equatable实现
  • 没有关联类型的枚举
  • 只拥有遵守Equatable协议关联类型的枚举
  • 只拥有遵守Equatable协议存储属性的结构体
  • 引用类型比较存储的地址值是否相等（是否引用着同一个对象），使用恒等运算符 ===、 !==

struct Point1 : Equatable {
    var x: Int, y: Int
}

var p1 = Point1(x: 10, y: 20)
var p2 = Point1(x: 11, y: 22)
print(p1 == p2) // false
print(p1 != p2) // true

4.Comparable

要想比较2个实例的大小，一般做法是:

• 遵守 Comparable 协议
• 重载相应的运算符

/// score大的比较大，若score相等，age小的比较大
struct Student : Comparable {
    var age: Int
    var score: Int
    init(score: Int, age: Int) {
        self.score = score
        self.age = age
    }
    
    static func < (lhs: Student, rhs: Student) -> Bool {
        (lhs.score < rhs.score) || (lhs.score == rhs.score && lhs.age > rhs.age)
    }
    static func > (lhs: Student, rhs: Student) -> Bool {
        (lhs.score > rhs.score) || (lhs.score == rhs.score && lhs.age < rhs.age)
    }
    static func <= (lhs: Student, rhs: Student) -> Bool {
        !(lhs > rhs)
    }
    static func >= (lhs: Student, rhs: Student) -> Bool {
        !(lhs < rhs)
    }
}

var stu1 = Student(score: 100, age: 20)
var stu2 = Student(score: 98, age: 18)
var stu3 = Student(score: 100, age: 20)
print(stu1 > stu2)  //true
print(stu1 >= stu2) //true
print(stu1 > stu3)  //true
print(stu1 <= stu3) //true
print(stu2 < stu1)  //true
print(stu2 <= stu1) //true

5.自定义运算符

• 可以自定义新的运算符：在全局作用域使用operator进行声明
• prefix operator 前缀运算符
• postfix operator 后缀运算符
• infix operator 中缀运算符 ： 优先级组

/*
precedencegroup 优先级组 {
    associativity: 组合性(left\right\none)
    higherThan: 比谁的优先级高
    lowerThan: 比谁的优先级低
    assignment: true 代表在可选链操作中拥有跟赋值运算符一样的优先级
}
 */

prefix operator +++
infix operator +- : PlusMinusPrecedence
precedencegroup PlusMinusPrecedence {
    associativity: none
    higherThan: AdditionPrecedence
    lowerThan: MultiplicationPrecedence
    assignment: true
}

struct TestPoint {
    var x: Int, y: Int
    static prefix func +++ (point: inout TestPoint) -> TestPoint {
        point = TestPoint(x: point.x + point.x, y: point.y + point.y)
        return point
    }
    static func +- (left: TestPoint, right: TestPoint) -> TestPoint {
        return TestPoint(x: left.x + right.x, y: left.y - right.y)
    }
    static func +- (left: TestPoint?, right: TestPoint) -> TestPoint {
        print("+-")
        return TestPoint(x: left?.x ?? 0 + right.x, y: left?.y ?? 0 - right.y)
    }
}

struct Person {
    var point: TestPoint
}
var person: Person? = nil
person?.point +- TestPoint(x: 10, y: 20)