Challenge 2: An expect class — Solution

// ── commonMain/kotlin/Logger.kt ──
expect class Logger() {
    fun log(message: String)
}

fun logTestMessage(logger: Logger) {
    logger.log("This is a test message")
}

// ── androidMain/kotlin/Logger.kt ──
actual class Logger {
    actual fun log(message: String) {
        println("[Android] $message")
    }
}

// ── iosMain/kotlin/Logger.kt ──
actual class Logger {
    actual fun log(message: String) {
        println("[iOS] $message")
    }
}

fun main() {
    // Standing in for whichever platform's actual Logger would really be
    // compiled in — here just constructing one directly to demonstrate.
    val logger = Logger()
    logTestMessage(logger)
}

/*
Output (using the androidMain actual, for example):
[Android] This is a test message

Notes:
- "expect class Logger()" declares both the class and its constructor
  shape in commonMain, with the actual behavior of log() left to each
  platform.
- Each actual class Logger provides a genuinely different
  implementation — different log prefix — while presenting the exact
  same public shape (a no-arg constructor, a log(String) function) that
  commonMain code depends on.
- logTestMessage() is written once in commonMain and works correctly no
  matter which platform's actual Logger ends up compiled in — it never
  branches on platform itself.
*/