I have a question about a behavior around `value class`es. From my understanding they’re (in most ca...

# getting-started

[Saharath Kleips]

I have a question about a behavior around `value class`es. From my understanding they’re (in most cases) a zero-overhead way to make a “type-safe primitive”. But what happens in the case of collections and moving between the primitive and the value class?

value class Box(val str: String)
val letters: List<String> = listOf("A","B","C",...)

val boxes: List<Box> = letters.map(::Box)
val backToLetters: List<String> = boxes.map(Box::str)

Are there any costs associated with moving between a List<String> and List<Box> and vice versa?

[ephemient]

yes, that boxes/unboxes each element, as of it were a non-value class

[Zak Henry]

since the value itself would be inlined, wouldn’t the identity map be optimised away by the compiler resulting in a no-op?

[ephemient]

no, inline classes have two representations

[ephemient]

https://kotlinlang.org/docs/inline-classes.html#representation

[Zak Henry]

ah interesting thanks 👍

[CLOVIS]

Note that this is the current behavior, but there are plans to eliminate those boxes in the future (thanks for Project Valhalla, for example)

[Stephan Schroeder]

you know that you forgot the @JvmInline infront of value class , don't you!?