How does the reified keyword in Kotlin work?

Understanding reified types

Generics

While using generics in Kotlin, we can perform operations on a value of any type T:

fun <T> doSomething(value: T) { println("Doing something with value: $value") // OK } 

Here we are implicitly calling the toString() function of the value and that works.

But we can't perform any operations on the type T directly:

fun <T> doSomething(value: T) { println("Doing something with type: ${T::class.simpleName}") // Error } 

Let's understand the reason for this error.

Type erasure

In the code above, the compiler gives an error: Cannot use 'T' as reified type parameter. Use a class instead. This happens because at compile time, the compiler removes the type argument from the function call.

For example, if you call the function as:

doSomething<String>("Some String") 

The compiler removes the type argument part <String> and all that's left at the runtime is:

doSomething("Some String") 

This is called type erasure. So, at runtime (inside the function definition), we cannot possibly know exactly which type the T stands for.

Java solution

The solution to this type erasure problem in Java was to pass an additional argument specifying the type with the Class (in Java) or KClass (in Kotlin):

fun <T: Any> doSomething(value: T, type: KClass<T>) { println("Doing something with type: ${type.simpleName}") // OK } 

This way our code is not affected by type erasure. But this solution is verbose and not very elegant since we have to declare it as well as call it with an additional argument. Also, specifying the type bound Any is mandatory.

Type reification

The best solution to the problem above is type reification in Kotlin. The reified modifier before the type parameter enables the type information to be retained at runtime:

inline fun <reified T> doSomething(value: T) { println("Doing something with type: ${T::class.simpleName}") // OK } 

In the code above, thanks to the reified type parameter, we no longer get the error while performing an operation on type T. Let's see how inline functions make this magic possible.

inline functions

When we mark a function as inline, the compiler copies the actual body of that inline function wherever that function is called. Since we marked our doSomething() function as an inline, the following code:

fun main() { doSomething<String>("Some String") } 

gets compiled to:

fun main() { println("Doing something with type: ${String::class.simpleName}") } 

So, the two code snippets shown above are equivalent.

While copying the body of an inline function, the compiler also replaces the type parameter T with the actual type argument that is specified or inferred in the function call. For example, notice how the type parameter T is replaced with the actual type argument String.

Type checking and type casting of reified types

The main objective of a reified type parameter is to know the exact type that the type parameter T represents at runtime.

Let's say we have a list of different types of fruits:

val fruits = listOf(Apple(), Orange(), Banana(), Orange()) 

And we want to filter all the Orange types in a separate list like following:

val oranges = listOf(Orange(), Orange()) 

Without reified

For filtering the fruit types, we may write an extension function on List<Any> like following:

fun <T> List<Any>.filterFruit(): List<T> { return this.filter { it is T }.map { it as T } // Error and Warning } 

In this code, first we filter the types and only take the element if its type matches the given type argument. Then we cast each element to the given type argument and return the List. But there are two problems.

Type checking

While type checking it is T, we are introduced to another error by the compiler: Cannot check for instance of erased type: T. This is another kind of error you may come across due to type erasure.

Type casting

While type casting it as T, we are also given a warning: Unchecked cast: Any to T. The compiler cannot confirm the type due to type erasure.

reified types to the rescue

We can easily overcome these two problems by marking the function as inline and making the type parameter reified as explained earlier:

inline fun <reified T> List<Any>.filterFruit(): List<T> { return this.filter { it is T }.map { it as T } } 

And then call it like following:

val oranges = fruits.filterFruit<Orange>() 

I showed this function for easier demonstration. For the purpose of filtering the types in collections, there is already a standard library function filterIsInstance(). This function has used the inline and reified modifiers in the similar manner. You can simply call it as following:

val oranges = fruits.filterIsInstance<Orange>() 

Passing reified parameter as an argument

The reified modifier makes it possible for a function to pass the type parameter as a type argument to another function that has reified modifier:

inline fun <reified T> doSomething() { // Passing T as an argument to another function doSomethingElse<T>() } inline fun <reified T> doSomethingElse() { } 

Getting the generic type of the reified type

Sometimes a type argument can be a generic type. For example, List<String> in the function call doSomething<List<String>>(). It's possible to know this entire type, thanks to reification:

inline fun <reified T> getGenericType() { val type: KType = typeOf<T>() println(type) } 

Here the typeOf() is a standard library function. The println() function above will print kotlin.collections.List<kotlin.String>, if you call the function as getGenericType<List<String>>(). The KType includes KClass, type argument information and nullability information. Once you know the KType, you can perform reflection on it.

Java interoperability

The inline functions declared without reified type parameters can be called from Java as regular Java functions. But the ones declared with the reified type parameters are not callable from Java.

Even if you call it using the reflection like following:

Method method = YourFilenameKt.class.getDeclaredMethod("doSomething", Object.class); method.invoke("hello", Object.class); 

You get the UnsupportedOperationException: This function has a reified type parameter and thus can only be inlined at compilation time, not called directly.

Conclusion

In many cases, the reified types help us get rid of the following errors and warnings:

1. Error: Cannot use 'T' as reified type parameter. Use a class instead.
2. Error: Cannot check for instance of erased type: T
3. Warning: Unchecked cast: SomeType to T

The purpose of reified is to allow the function to use T at compile time (to access it within the function).

For example:

inline fun <reified T:Any> String.convertToObject(): T{ val gson = Gson() return gson.fromJson(this,T::class.java) } 

To use:

val jsonStringResponse = "{"name":"bruno" , "age":"14" , "world":"mars"}" val userObject = jsonStringResponse.convertToObject<User>() println(userObject.name) 

If you need the type of your generic parameter in your function, you have to make it reified.

Since the compiler has to replace the generic type to get its type, it will also ask you for adding inline keyword.