Inline functions

Learn performance safety with inline and noinline.

What does inline do?

The inline keyword tells the compiler to copy a function body into its call site. This reduces the lambda allocation overhead in higher-order functions.

Example: inline fun forEachTwice(items: List<Int>, action: (Int) -> Unit). Here the action lambda is not allocated each call; it is inlined into the body. This improves performance but grows the bytecode. For long bodies, avoid inline. It is ideal for short, lambda-parameter functions.

noinline and crossinline

Sometimes you do not want a lambda parameter to be inlined. Then use the noinline marker: noinline action: () -> Unit.

Sometimes an inlined lambda does a non-local return, which is a problem in some contexts. Use crossinline to prevent it. Example: when passing a lambda to another function inside an inline function, use crossinline. Both keywords only apply to parameters of inline functions.

Exercise Write inline forEachTwice

Define inline fun forEachTwice(items: List<Int>, action: (Int) -> Unit). It should call action twice for each item. In main, call it with listOf(1, 2) and use println(it) as the action.

Show example solution

inline fun forEachTwice(items: List<Int>, action: (Int) -> Unit) {
    for (i in items) {
        action(i)
        action(i)
    }
}
fun main() {
    forEachTwice(listOf(1, 2)) { println(it) }
}

Test yourself

Add the inline keyword

____ fun twice(action: () -> Unit) {
    action()
    action()
}

Show answer

inline fun twice(action: () -> Unit) {
    action()
    action()
}

Match inline keywords

inline

Copies the body
noinline

Do not copy parameter
crossinline

Blocks local return
fun

Function declaration
When is an inline function preferred?
- For short, lambda-parameter, frequently called functions
- Only for abstract class methods
- For long, rarely called functions
- Only for non-inlineable generic functions
Show answer

For short, lambda-parameter, frequently called functions — inline shines for short, frequently called lambda-parameter functions.

Next lesson: reified

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies