operator 将一个函数标记为重载一个操作符或者实现一个约定

表达式	翻译为
+a	a.unaryPlus()
-a	a.unaryMinus()
!a	a.not()
a + b	a.plus(b)
a - b	a.minus(b)
a * b	a.times(b)
a / b	a.div(b)
a % b	a.rem(b)、 a.mod(b) （已弃用）
a..b	a.rangeTo(b)
a in b	b.contains(a)
a !in b	!b.contains(a)
a == b	a?.equals(b) ?: (b === null)
a != b	!(a?.equals(b) ?: (b === null))
a > b	a.compareTo(b) > 0
a < b	a.compareTo(b) < 0
a >= b	a.compareTo(b) >= 0
a <= b	a.compareTo(b) <= 0
a()	a.invoke()
a(i)	a.invoke(i)
a(i, j)	a.invoke(i, j)
a(i_1, ……, i_n)	a.invoke(i_1, ……, i_n)

unaryMinus

data class Point(val x: Int, val y: Int)

operator fun Point.unaryMinus() = Point(-x, -y)

val point = Point(10, 20)

fun main() {
   println(-point)  // 输出“Point(x=-10, y=-20)”
}

plus

data class Counter(val dayIndex: Int) {
    operator fun plus(increment: Int): Counter {
        return Counter(dayIndex + increment)
    }
}

invoke

class Hello{
    operator fun invoke(target: String) {
        say()
        println(target)
    }

    fun say() {
        println("hello ")
    }
}

fun main() {
    val hello = Hello()
    hello("world")
}