What happens if an abstract function is not implemented in a subclass?

When a subclass fails to implement an abstract function, it results in a compilation error. In Kotlin, abstract functions are defined in abstract classes, and these functions must be overridden in any concrete subclass. If a subclass does not provide an implementation for an abstract method, the compiler cannot guarantee the subclass behaves as expected, leading to a situation where the code cannot compile successfully.

This strict requirement ensures that all the necessary behaviors defined by the abstract class are concretely available in the subclass, maintaining the integrity of the class hierarchy. Thus, any attempt to instantiate a subclass that misses implementing the abstract function will result in a compilation error, signaling to the developer that further action is needed to complete the subclass properly.