Mastering Kotlin Inline Functions: A Deep Dive into inline, noinline, and crossinline

In Kotlin, inline, crossinline, and noinline are keywords used when working with higher-order

functions (functions that take other functions as parameters or return functions).

1. inline

The inline keyword is used to request the compiler to inline the function body, meaning the compiler replaces the function call with its actual body at the call site. This is particularly useful for higher-order functions to reduce the overhead of creating function objects and performing lambda calls.

Use Case

• Reduces runtime overhead caused by lambda expressions and function calls.
• Particularly useful in performance-critical applications.

Example:

Compiled Output: The lambda is directly inlined into the inlineFunction‘s call site:

2. noinline

The noinline keyword is used to prevent inlining for specific lambda parameters in an inline function. Normally, all lambda parameters in an inline function are inlined, but with noinline, you can exclude certain lambdas.

Use Case

• Useful when you need to pass the lambda as a first-class object (e.g., to store or pass it elsewhere), which cannot happen if it’s inlined.

Example:

Here, the action lambda is not inlined and can be treated like a normal function object.

3. crossinline

The crossinline keyword is used to mark a lambda parameter in an inline function that must not return (i.e., it can’t use return to exit the enclosing function). This is often required when the lambda is passed to another function or stored.

Use Case

• Ensures the lambda can’t cause unexpected behavior by returning from the enclosing function.

Example:

If you attempt to use return inside the lambda, the compiler will throw an error because the lambda is passed to another context (in this case, a Runnable).

Summary of Differences