Problem Statement
What are inline functions in Kotlin and when should you use them?
Explanation
Inline functions are marked with the inline keyword causing the compiler to copy the function body to the call site instead of creating a function call, eliminating the overhead of lambda objects and function calls which is especially important for higher-order functions. When you inline a function that takes lambdas as parameters, the lambda code is also inlined avoiding lambda object creation and virtual calls.
Use inline for small higher-order functions that are called frequently to avoid performance overhead, enable non-local returns from lambdas allowing return from the enclosing function, and use noinline parameter modifier when you need to store a lambda parameter or call it from a nested function. Use crossinline when you need to prevent non-local returns but still want inlining benefits.
Don't overuse inline as it increases code size since the function body is duplicated at each call site, and avoid inlining large functions or functions with complex control flow. Inline is most beneficial for collection operations and other higher-order functions used in hot code paths where performance matters.