In Kotlin, reified types are used with inline functions to enable type information to be available at runtime. Normally, type parameters in generics are erased at runtime due to type erasure, but reified allows the type to remain available for reflective operations or type-specific logic.

Here are the key points to use reified types with inline functions:

1. Mark the function as inline: The function must be an inline function for it to use reified types.
2. Use the reified modifier: The generic type parameter must be marked as reified to prevent type erasure and keep the type information available at runtime.
3. Call the function without explicitly passing the type: The compiler will infer the type automatically.

Here’s an example that demonstrates using reified types:

Example: Filtering a List Based on Type

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

fun main() {
    val mixedList: List<Any> = listOf(1, "Hello", 2.5, "World", 42)

    // Use the reified function to filter only Strings
    val strings: List<String> = mixedList.filterByType()
    println(strings) // Output: [Hello, World]

    // Use the reified function to filter only Integers
    val integers: List<Int> = mixedList.filterByType()
    println(integers) // Output: [1, 42]
}

Explanation:

1. Inline Functions: Inline functions replace the function body at the call site, enabling the type information to persist after type erasure.
2. Reified Modifier: When you use reified T, you can check the type (it is T) or even fetch its class (T::class) at runtime because the type information is preserved.
3. Flexible Filtering: In the above example, the filter function dynamically determines the type of each element and includes the matching elements in the resulting list.

When to Use Reified Types

• When you need access to the type at runtime (e.g., to perform type checking or reflection).
• When working with generic functions that act differently based on the type parameter.

Note:

• You can only use reified with inline functions.
• Avoid overusing inline functions, as they can increase code size due to function duplication at each call site.