In Kotlin, is it possible to overload the following operation to x: X?

x[i]  = j

Currently, I can only see an indirect way, like defining some X.get that returns an object of type XAtIndex with a reference to the original, then defining XAtIndex.plugAssign that modifies the original.

CodePudding user response：

If you want a = that mutates an object instead of changing what is stored in a variable, you must implement plusAssign on the type of the object returned by get(). An example of this in the standard library is MutableList.plusAssign().

If you want the more traditional behavior of reassigning the value held at an index after creating a modified copy, your class should have matching get and set operator functions. Then you can implement a plus function on whatever is the type of the get/set (if it doesn't have one). When = is used, it will use the getter and setter operator functions along with the plus operator of the type returned by get. Example:

class Foo {
    private var thing1: String = "Hello"
    private var thing2: String = "World"

    operator fun get(thing: Int) = when (thing) {
        1 -> thing1
        2 -> thing2
        else -> throw IllegalArgumentException()
    }.also { println("get") }

    operator fun set(thing: Int, value: String) {
        when (thing) {
            1 -> thing1 = value
            2 -> thing2 = value
            else -> throw IllegalArgumentException()
        }
        println("set")
    }

    override fun toString(): String ="Foo(thing1='$thing1', thing2='$thing2')"

}

fun main() {
    val foo = Foo()
    foo[1]  = "!!!"
    println(foo)
}