assign variable only if it is null - kotlin

Question

on Ruby one have something like this:
#var ||= 'value'
basically, it means that #var will be assigned 'value' only if #var is not assigned yet (e.g. if #var is nil)
I'm looking for the same on Kotlin, but so far, the closest thing would be the elvis operator. Is there something like that and I missed the documentation?

Answer 1

The shortest way I can think of is indeed using the elvis operator:
value = value ?: newValue
If you do this often, an alternative is to use a delegated property, which only stores the value if its null:
class Once<T> {
private var value: T? = null
operator fun getValue(thisRef: Any?, property: KProperty<*>): T? {
return value
}
operator fun setValue(thisRef: Any?, property: KProperty<*>, value: T?) {
this.value = this.value ?: value
}
}
You can now create a property that uses this like so:
var value by Once<String>()
fun main(args: Array<String>) {
println(value) // 'null'
value = "1"
println(value) // '1'
value = "2"
println(value) // '1'
}
Note that this is not thread-safe and does not allow setting back to null. Also, this does evaluate the new expression while the simple elvis operator version might not.

Answer 2

Other way we could do it is by using ifEmpty..
From the docs: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.text/if-empty.html
Usage:
val empty = ""
val emptyOrNull: String? = empty.ifEmpty { null }
println(emptyOrNull) // null
val emptyOrDefault = empty.ifEmpty { "default" }
println(emptyOrDefault) // default
val nonEmpty = "abc"
val sameString = nonEmpty.ifEmpty { "def" }
println(sameString) // abc
EDIT:
Seems like this does not work if the initial value is NULL and only for strings..