reified

Learn runtime type safety with reified.

What is reified?

Normally inside a generic function you cannot access the runtime type of T because the JVM applies type erasure. reified bypasses that.

If you mark a type parameter of an inline function as reified, the compiler substitutes the actual type at the call site. Example: inline fun <reified T> typeName(): String = T::class.simpleName ?: "?". Then typeName<Int>() returns "Int". reified only works in inline functions.

Filtering with reified

One common use of reified is to filter items of a specific type from a list.

Example: inline fun <reified T> List<Any>.filterOfType(): List<T> = filter { it is T }. Here the it is T check only works with reified because normally T is erased. Call: list.filterOfType<Int>(). This pattern appears in the Kotlin standard library too.

Exercise Write reified typeName

Write inline fun <reified T> typeName(): String. It should return T::class.simpleName ?: "?". In main, print the results of typeName<Int>() and typeName<String>() on separate lines.

Show example solution

inline fun <reified T> typeName(): String = T::class.simpleName ?: "?"
fun main() {
    println(typeName<Int>())
    println(typeName<String>())
}

Test yourself

Match reified parts

inline

Required base
reified

Type parameter marker
T::class

Runtime type
simpleName

Class name
Which keyword must reified work with?
- suspend
- inline
- tailrec
- operator
Show answer

inline — reified only applies in inline functions.

Complete the reified typeName function

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

Show answer

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

Next lesson: tailrec

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