17 | 赋值和算数运算符-Swift核心技术与实战-极客时间

不好意思，前几天学所有权在消化做笔记。今天把这段代码分享给老师和大家，代码来源于互联网，注释为本人注释，如有不对的地方请大家指正，。 prefix operator +☀ infix operator +-☀ : PlusMinusPrecedence precedencegroup PlusMinusPrecedence { associativity: none // 结合性(left\right\none) higherThan: AdditionPrecedence //比谁的优先级高 lowerThan: MultiplicationPrecedence//比谁的优先级低，不能是该模块内部的 assignment: false //如果设置成true，那么遇到可选链的时候该操作符就相当于赋值操作符。否则false就不执行赋值操作。 } struct Point { var x: Int, y: Int static prefix func +☀(point: inout Point) -> Point { point = Point(x: point.x + point.x, y: point.y + point.y) return point } static func +-☀(left: Point,right: Point) -> Point { return Point(x: left.x + right.x, y: left.y - right.y) } static func +-☀(left: Point?,right: Point) -> Point { print("+-☀") return Point(x: left?.x ?? 0 + right.x, y: left?.y ?? 0 - right.y) } } struct Person { var point: Point //= Point(x: 10, y: 20) } var person: Person? //= Person.init() person?.point +-☀ Point(x: 10, y: 20)//(Point(x: 10, y: 20) +-☀ Point(x: 10, y: 20)) print(person?.point as Any) var test = Point(x: 10, y: 20) Point(x: 10, y: 20) +-☀ (+☀test)

作者回复: 我把你的例子改了一下，既然是赋值运算，你的实现里明显不是。 infix operator +-☀ : PlusMinusPrecedence precedencegroup PlusMinusPrecedence { associativity: none higherThan: AdditionPrecedence lowerThan: MultiplicationPrecedence assignment: true } struct Point { var x: Int, y: Int static func +-☀(left: inout Point, right: Point) { left = Point(x: left.x + right.x, y: left.y - right.y) } } struct Person { var point: Point } var person: Person? person?.point +-☀ Point(x: 10, y: 20) print(person?.point as Any) 你可以尝试把assignment的true改成false，是会报错的，所以assignment的可选链里的作用如下： “This proposal quietly drops the assignment modifier that exists on operators today. This modifier had one important function–an operator marked assignment gets folded into an optional chain, allowing foo?.bar += 2 to work as foo?(.bar += 2) instead of failing to type-check as (foo?.bar) += 2. In practice, all Swift operators currently marked assignment are at the equivalent of the Assignment precedence level, so the core team recommends making this optional chaining interaction a special feature of the Assignment precedence group.”

2019-11-21

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Life is fantastic

高级运算符文档中，在定义优先级组的时候有个布尔值的属性assignment，设置true的时候有什么讲究。看了人家写的代码可选链为空的时候做赋值运算返回nil，而不是中缀运算了。可以看出这个中缀自定义的运算符相当于赋值运算符了，并不会调用左边形式参数为可选类型的自定义运算。

作者回复: assignment只是表明是否是赋值，你这个有代码么，拿出来看一下

2019-09-10

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Geek_zoey

老师生病了呀，保重身体

2019-08-29

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邦邦

＝不返回值挺好，也不能做连续赋值了！

2020-02-16

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