39. Reified Type Parameters

On the JVM generic types are erased at runtime, so you normally can’t write T::class or is T. Marking an inline function’s parameter reified keeps the concrete type available at runtime.

// 'reified' keeps the type at runtime; normally generics are erased
inline fun <reified T> List<*>.filterByType(): List<T> =
    filterIsInstance<T>()

inline fun <reified T> typeName(): String = T::class.simpleName ?: "?"

fun main() {
    val mixed = listOf(1, "two", 3, "four", 5.0)
    println(mixed.filterByType<String>())
    println(mixed.filterByType<Int>())
    println("T is ${typeName<Double>()}")
}

Because the function is inlined at each call site, the compiler can substitute the real type — so T::class and is T checks (here via filterIsInstance) work. reified is only allowed on inline functions.

Running it:

$ kotlin run
[two, four]
[1, 3]
T is Double

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