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Using Inline Functions in Kotlin

Kotlin, Inline Functions1 min read

Inline functions are a common feature of modern programming languages, including Kotlin. They allow developers to write more concise and efficient code by inlining the function's body at the call site instead of creating a new function call. In this article, we'll explore what inline functions are, how they work, and when to use them.

What are Inline Functions?

An inline function is a function whose code is copied directly into the calling code during compilation, rather than executing a separate function call at runtime. This means that the calling code is executed faster because there's no overhead associated with invoking a function call.

In Kotlin, inline functions are marked with the inline keyword before the fun keyword:

1inline fun calculateSum(a: Int, b: Int): Int {
2 return a + b

How Do Inline Functions Work?

When you call an inline function, its code is copied directly into the calling code, as if the function had never existed. This is called function inlining. The compiler does this by replacing the function call with the actual code of the function, which is then executed directly.

This process is different from regular function calls, where the function code is executed separately from the calling code. Inlining can save time and memory because it avoids the overhead associated with setting up a new stack frame for each function call.

When to Use Inline Functions

Inline functions are most useful for small functions that are called frequently, such as getters and setters or simple utility functions. Larger functions are not good candidates for inlining because their code can bloat the calling code, potentially slowing it down.

Inline functions are also useful when working with higher-order functions, such as lambda expressions or function references. In these situations, using an inline function can reduce the overhead associated with creating a new object for each function call.


Inline functions are a powerful tool for optimizing code and improving performance in Kotlin. By copying the function's code directly into the calling code during compilation, inline functions can speed up execution and reduce memory usage. Use inline functions when you need to repeatedly call a small function or when working with higher-order functions.