What is the difference between the expect/actual mechanism and standard interfaces in programming?

The expect/actual mechanism is used in Kotlin Multiplatform to define platform-dependent code. The key difference from standard interfaces is that:

• expect declares an API without implementation (for example, a function or class), which expects a platform-specific implementation.
• actual provides a concrete implementation for a specific platform (JVM, JS, Native).

Unlike interfaces, which define a contract and require implementation in classes, expect/actual separates declaration and implementation across platforms, allowing you to write common code with platform-dependent details.

Example:

// commonMain
expect fun getPlatformName(): String

// androidMain
actual fun getPlatformName() = "Android"

// iosMain
actual fun getPlatformName() = "iOS"

Thus, expect/actual helps create multiplatform libraries with different implementations for each platform, whereas interfaces simply define a contract for classes within a single platform.