CODEWITHSUNDEEP
genericskotlinkotlin-reified-type-parameters
George Hawkins
George Hawkins

Reputation: 38784

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

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.

Upvotes: 10

Views: 2142

Answers (2)

Alexey Romanov
Alexey Romanov

Reputation: 170745

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.

Upvotes: 0

Nikky
Nikky

Reputation: 518

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

Upvotes: 3

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