Kotlin how to copy class property fields in its own member function - kotlin

Question

I have the following code, I would like to update the properties in class A, in c++, I can use something like this = A() within the udpate() that invokes implicity operator = ()
data class B(
var A: Int = 10,
var B: Long = 20L,
var C: Double = 200.0
)
data class A(
var a: Int = 0,
var b: Long = 0L
) {
fun badUpdate(input: B) {
var holder = this#A // It does not work, any better way to update within the member function?
holder = A(input.A, input.B)
}
fun aboriginalUpdate(input:B) {
a = input.A // It works, but what if I have 30 properties?
b = input.B
}
fun update(input: B) : A {
return A(input.A, input.B) // Works but not ideal, the return value is not necessary
}
}
fun main() {
var data0 = A()
val data = B(A = 300)
data0.badUpdate(data) // data0.a does not change (nothing invoked in update0)
data0 = data0.update(data)
println("$data0")
}
Any better way to update the properties within the class A？

Answer 1

I thnk 'apply()' is what you are looking for:
data class B(
var a: Int = 10,
var b: Long = 20L,
var c: Double = 200.0
)
data class A(
var a: Int = 0,
var b: Long = 0L
) {
fun goodUpdate(input: B) {
this.apply {
a = input.a
b = input.b
}
}
}
fun main() {
var data0 = A()
val data = B(a = 300)
data0.goodUpdate(data)
println("$data0") // A(a=300, b=20)
}
but unlike in the example you provided from C, you are not creating a new instance, you are updating properties of the existing one, which I believe you wanted
but you can achieve that even simpler, by reassigning object fields:
fun betterUpdate(input: B) {
this.a = input.a
this.b = input.b
}