Kotlinhard

What are reified generics and why do they require `inline`?

Tags

#reified#inline#generics

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Answer

Because of type erasure, generic type info isn’t available at runtime. `reified` in an `inline` function keeps the type at the call site, so you can do things like `T::class` or `value is T` safely.

inline fun <reified T> Any?.asTypeOrNull(): T? = this as? T

val x: Any = 123
val n: Int? = x.asTypeOrNull<Int>()

Advanced answer

Deep dive

On the JVM, generics are erased at runtime (you can’t normally do `value is T`).

`reified` + `inline` works because the compiler substitutes the function body at the call site, so it can embed the concrete type info.

Example

inline fun <reified T> Any?.asOrNull(): T? = this as? T

val x: Any = 1
val i: Int? = x.asOrNull<Int>()

Common pitfalls

Thinking reified removes type erasure globally (it’s only for inline call sites).
Using it where you still need full generic info (e.g., `List<T>` element type at runtime is still tricky without type tokens).

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