Calling a fun with reified type parameter if you just have a KClass object?

Question

One can only use reified type parameters with inline functions. So if I want such a parameter for a class I need a trick like this:

class Foo<T : Any>(private val clazz: KClass<T>) {
    companion object {
        inline fun <reified T: Any> create() = Foo(T::class)
    }
}

I can then create instances of Foo like this:

val foo = Foo.create<Bar>()

Within Foo I have access clazz but my question is can I then use clazz when I need to call methods that require a reified type parameter`?

E.g. within Foo I'd like to add a method like this:

fun print(list: List<Alpha>) {
    list.filterIsInstance<T>().forEach { print(it) }
}

But as far as I can see there's no way to get from clazz to something I can use as a type parameter here.

And yes, I know there's a form of filterIsInstance that takes a Class so I can do:

list.filterIsInstance(clazz.java).forEach { print(it) }

However many libraries contain methods where both forms (explicit class parameter and reified type parameter) are not provided.

E.g. the Jackson Kotlin Extensions.kt. Actually this isn't a great example as the non-reified equivalents are all one-liners but this isn't always the case - then you end up unpacking the implementation of the reified-type-parameter method into your code.

Given the description of how reification works in this other [SO answer](https://stackoverflow.com/a/45952201) it seems clear anything to do with `reified` is resolved at _compile_ time so you cannot hope to do anything with a `KClass` or `Class` object that you've acquired at runtime (all methods involving `reified` have been inlined at this point and essentially don't exist in any real sense at runtime). — George Hawkins, Apr 05 '19 at 14:40

score 1 · Accepted Answer · answered Mar 28 '19 at 15:06

no, because those functions are inline, they are inlined at compiletime and a Class or KClass is using reflection at runtime

there are some tricks that you can do.. like with the companion class, but that does nto need the KClass<T> at all.. anything else that provides a generic argument of T would work just as well for the reified type info

PS: reflection also cannot help you reliably because inline functions do not really exist at runtime, as explained by their modifier inline

score 0 · Answer 2 · answered Mar 28 '19 at 17:39

Unless I am missing something, everything you can do with T in a function with reified T can be translated to a use of KClass: e.g. x is T becomes clazz.isInstance(x), x as T becomes clazz.cast(x), calls of other functions with reified type parameters are translated recursively, etc. Since the function has to be inline, all APIs it uses are visible at the call site so the translation can be made there.

But there's no automatic way to do this translation, as far as I know.

Calling a fun with reified type parameter if you just have a KClass object?

2 Answers2