Reified

Learn runtime type info with reified.

Reified

reified makes a generic type parameter accessible at runtime. Normally generic type parameters are erased (type erasure); reified bypasses this.

Example: inline fun <reified T> typeName(): String { return T::class.simpleName ?: "Unknown" }. This function is called as typeName<Int>() and returns "Int". reified only works with inline functions, because the type information is inlined into the call site. This enables runtime checks with generic types.

The typeName function

A reified generic function accesses the type parameter's class information. T::class gives the class reference.

Example: inline fun <reified T> typeName(): String { return T::class.simpleName ?: "Unknown" }. Calling typeName<Int>() returns "Int". T::class is Kotlin reflection; .simpleName returns the short class name. ?: "Unknown" provides null safety. reified is useful for generic type checks and filtering.

Exercise The typeName reified function

Define the function inline fun <reified T> typeName(): String whose body returns T::class.simpleName ?: "Unknown". In main call println(typeName<Int>()).

Show example solution

inline fun <reified T> typeName(): String {
    return T::class.simpleName ?: "Unknown"
}

fun main() {
    println(typeName<Int>())
}

Test yourself

Match the reified parts

inline fun <reified T>

inline and reified signature
T::class

Class reference
.simpleName

Simple name
typeName<Int>()

Call
Why is <code>reified</code> used with an <code>inline</code> function?
```
reified cannot work without inline; the type information must be inlined.
```
- Because inline is faster
- To avoid a syntax error without inline
- To inline the type information at the call site
- Because reified can only be called with inline
Show answer

To inline the type information at the call site — inline inlines the type information at the call site, making reified possible.

Complete the reified definition

inline fun <____ T> typeName(): String {
    return T::class.simpleName ?: "Unknown"
}

Show answer

inline fun <reified T> typeName(): String {
    return T::class.simpleName ?: "Unknown"
}

Next lesson: Summary - Generics

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