Humm... is there any way to allow passing a `value class WriteableResource(val ro: Resource)` to a...

# getting-started

[Daniel Svensson]

Humm... is there any way to allow passing a value class WriteableResource(val ro: Resource) to a method only accepting Resource without explicitly passing its member? Does it have to explicitly implement an interface for Resource?

[Daniel Svensson]

I suspect this answers that question 😞

interface I

@JvmInline
value class Foo(val i: Int) : I
fun asInterface(i: I) {}
val f = Foo(42)
asInterface(f) // boxed: used as type I

[Daniel Svensson]

but at least it's possible to refer to the embedded object, so that brings compile time safety with no overhead, and any evildoings will be apparent in the code.

[thanksforallthefish]

with a regular class you could do

class WriteableResource(val ro: Resource) : Resource by ro

and you can then pass it to all methods accepting a Resource.

but this syntax is not supported by value classes

[Daniel Svensson]

Oh shiny, didn't know that. Neat in other cases.

[Daniel Svensson]

I wonder if the fact that the implemented interface == the embedded type allows for avoiding the boxing. I guess I have to check the generated bytecode at some point.

[Daniel Svensson]

@thanksforallthefish any idea if that's planned for value classes? if so, any issue to subscribe to?

[thanksforallthefish]

honestly I have no idea, I don’t see a real reason why delegation should not be possible with value classes (so there might be an issue to add it somewhere). I might be missing some basic though

anyway, if you find any issue let me know

[Daniel Svensson]

https://youtrack.jetbrains.com/issue/KT-27435

[thanksforallthefish]

thank you 🙂 it is fun how a single sentence can trigger a whole of reasoning

Currently, we should either save the result of expr somewhere...

kinda obvious why delegation for inlined classes is not “possible”, since inlined classes disappear at runtime I see how it would require workarounds. and given the change to value class gives a better semantic to what a value class should be for (imo, no behavior, just data) it probably makes sense to forbid a more behavior-oriented pattern such as delegation