2019-08-29老师生病了呀，保重身体 4
2019-11-21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)展开作者回复: 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.” 1- 2019-09-10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
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