What are inline classes in Kotlin?

Inline class is a special type of class in Kotlin that encapsulates the value of another type and does not create an additional container object at runtime unless used as a regular object. It helps prevent errors related to incorrect type usage while maintaining efficiency.

Main characteristics:

• Value-based: Instances behave more like values than objects.
• Wrapper Optimization: The compiler attempts to replace inline class instances with the underlying value, minimizing overhead.
• Type Safety: Provides strong typing for the underlying value.

Example:

inline class Password(val value: String) // Declaration of an inline class for hiding passwords

fun login(password: Password) {
    // Using inline class to ensure type safety
    println("Logging in with password: ${password.value}")
}

fun main() {
    val myPassword = Password("secure_password_123")
    login(myPassword) // Passing an inline class
    // val notAPassword: String = myPassword // Compilation error - cannot assign directly to the base type
}

Restrictions:

• Must have exactly one parameter in the primary constructor.
• Cannot have initializer blocks (init).
• Cannot have inner classes.
• Cannot extend other classes except interfaces.

Optimization occurs when using an inline class as a function parameter type or return type, as well as when accessing its members. If an inline class instance is stored in a variable or field, it can be represented by the underlying type if possible. If the instance is used polymorphically (e.g., as Any), it will be wrapped.