How to write in-place extension-function

Question

fun String.reversed():String{
// return another String..no 
prob
}

fun String.reverse(){
//in-place change this
} 

How to write the String.reverse()?

Answer 1

You can check how the in-place extension function reverse is implemented for MutableList in the Kotlin codebase: https://github.com/JetBrains/kotlin/blob/1.3.50/libraries/stdlib/js/src/generated/_CollectionsJs.kt#L20

fun <T> MutableList<T>.reverse(): Unit {
    val midPoint = (size / 2) - 1
    if (midPoint < 0) return
    var reverseIndex = lastIndex
    for (index in 0..midPoint) {
        val tmp = this[index]
        this[index] = this[reverseIndex]
        this[reverseIndex] = tmp
        reverseIndex--
    }
}

In general, you take this instance and call mutating methods on it to modify its content in place.

However, String class is immutable: it doesn't provide methods to modify its content in place, and, therefore, you cannot write an extension that modifies a string in place either.