expect/actual

Learn platform code with expect/actual.

What is expect/actual?

expect/actual is the mechanism in KMP that manages platform-specific code. expect declares a contract in commonMain; each target implements it with actual.

For example expect fun platformName(): String is written in commonMain. The iOS target defines actual fun platformName(): String = "iOS", the JVM target actual fun platformName(): String = "JVM". The compiler picks the right actual for each target. This way shared code can call the function without knowing the platform.

An actual example

An actual implementation is written in the target's source set. For the JVM target that is jvmMain, for iOS it is iosMain.

Example: actual fun platformName(): String = "JVM". The signature of the actual must match the expect exactly; the type, parameters and name must agree. Otherwise the compiler errors. This strictness keeps shared code consistent across targets. In this lesson we model the concept with a platformName function.

Exercise Write platformName

Define fun platformName(): String. It should return "JVM". In main, write println(platformName()).

Show example solution

fun platformName(): String = "JVM"
fun main() {
    println(platformName())
}

Test yourself

Match the expect/actual parts

expect fun

contract in commonMain
actual fun

impl in target set
expect val

constant in commonMain
actual val

value in target set
Where is the actual keyword written?
```
actual is target-specific.
```
- iosMain
- commonMain
- build.gradle
- test
Show answer

iosMain — actual is written in the target source set; e.g. iosMain.

Complete the actual declaration

____ fun log(msg: String) { println(msg) }

Show answer

actual fun log(msg: String) { println(msg) }

Next lesson: iOS target

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