when to use an inline function in Kotlin?

Question

Let's say you create a higher order function that takes a lambda of type () -> Unit (no parameters, no return value), and executes it like so:

fun nonInlined(block: () -> Unit) {
    println("before")
    block()
    println("after")
}

In Java parlance, this will translate to something like this (simplified!):

public void nonInlined(Function block) {
    System.out.println("before");
    block.invoke();
    System.out.println("after");
}

And when you call it from Kotlin...

nonInlined {
    println("do something here")
}

Under the hood, an instance of Function will be created here, that wraps the code inside the lambda (again, this is simplified):

nonInlined(new Function() {
    @Override
    public void invoke() {
        System.out.println("do something here");
    }
});

So basically, calling this function and passing a lambda to it will always create an instance of a Function object.

---

On the other hand, if you use the inline keyword:

inline fun inlined(block: () -> Unit) {
    println("before")
    block()
    println("after")
}

When you call it like this:

inlined {
    println("do something here")
}

No Function instance will be created, instead, the code around the invocation of block inside the inlined function will be copied to the call site, so you'll get something like this in the bytecode:

System.out.println("before");
System.out.println("do something here");
System.out.println("after");

In this case, no new instances are created.

Let me add: When not to use inline:

1. If you have a simple function that doesn't accept other functions as an argument, it does not make sense to inline them. IntelliJ will warn you:

   Expected performance impact of inlining '...' is insignificant. Inlining works best for functions with parameters of functional types

2. Even if you have a function "with parameters of functional types", you may encounter the compiler telling you that inlining does not work. Consider this example:

    inline fun calculateNoInline(param: Int, operation: IntMapper): Int {
        val o = operation //compiler does not like this
        return o(param)
    }
   

   This code won't compile, yielding the error:

   Illegal usage of inline-parameter 'operation' in '...'. Add 'noinline' modifier to the parameter declaration.

   The reason is that the compiler is unable to inline this code, particularly the operation parameter. If operation is not wrapped in an object (which would be the result of applying inline), how can it be assigned to a variable at all? In this case, the compiler suggests making the argument noinline. Having an inline function with a single noinline function does not make any sense, don't do that. However, if there are multiple parameters of functional types, consider inlining some of them if required.

So here are some suggested rules:

• You can inline when all functional type parameters are called directly or passed to other inline function
• You should inline when ^ is the case.
• You cannot inline when function parameter is being assigned to a variable inside the function
• You should consider inlining if at least one of your functional type parameters can be inlined, use noinline for the others.
• You should not inline huge functions, think about generated byte code. It will be copied to all places the function is called from.
• Another use case is reified type parameters, which require you to use inline. Read here.

Higher-order functions are very helpful and they can really improve the reusability of code. However, one of the biggest concerns about using them is efficiency. Lambda expressions are compiled to classes (often anonymous classes), and object creation in Java is a heavy operation. We can still use higher-order functions in an effective way, while keeping all the benefits, by making functions inline.

here comes the inline function into picture

When a function is marked as inline, during code compilation the compiler will replace all the function calls with the actual body of the function. Also, lambda expressions provided as arguments are replaced with their actual body. They will not be treated as functions, but as actual code.

In short:- Inline-->rather than being called ,they are replaced by the function's body code at compile time...

In Kotlin, using a function as a parameter of another function (so called higher-order functions) feels more natural than in Java.

Using lambdas has some disadvantages, though. Since they’re anonymous classes (and therefore, objects), they need memory (and might even add to the overall method count of your app). To avoid this, we can inline our methods.

fun notInlined(getString: () -> String?) = println(getString())

inline fun inlined(getString: () -> String?) = println(getString())

From the above example:- These two functions do exactly the same thing - printing the result of the getString function. One is inlined and one is not.

If you’d check the decompiled java code, you would see that the methods are completely identical. That’s because the inline keyword is an instruction to the compiler to copy the code into the call-site.

However, if we are passing any function type to another function like below:

//Compile time error… Illegal usage of inline function type ftOne...
 inline fun Int.doSomething(y: Int, ftOne: Int.(Int) -> Int, ftTwo: (Int) -> Int) {
    //passing a function type to another function
    val funOne = someFunction(ftOne)
    /*...*/
 }

To solve that, we can rewrite our function as below:

inline fun Int.doSomething(y: Int, noinline ftOne: Int.(Int) -> Int, ftTwo: (Int) -> Int) {
    //passing a function type to another function
    val funOne = someFunction(ftOne)
    /*...*/}

Suppose we have a higher order function like below:

inline fun Int.doSomething(y: Int, noinline ftOne: Int.(Int) -> Int) {
    //passing a function type to another function
    val funOne = someFunction(ftOne)
    /*...*/}

Here, the compiler will tell us to not use the inline keyword when there is only one lambda parameter and we are passing it to another function. So, we can rewrite above function as below:

fun Int.doSomething(y: Int, ftOne: Int.(Int) -> Int) {
    //passing a function type to another function
    val funOne = someFunction(ftOne)
    /*...*/
}

Note:-we had to remove the keyword noinline as well because it can be used only for inline functions!

Suppose we have function like this -->

fun intercept() {
    // ...
    val start = SystemClock.elapsedRealtime()
    val result = doSomethingWeWantToMeasure()
    val duration = SystemClock.elapsedRealtime() - start
    log(duration)
    // ...}

This works fine but the meat of the function’s logic is polluted with measurement code making it harder for your colleagues to work what’s going on. :)

Here’s how an inline function can help this code:

 fun intercept() {
    // ...
    val result = measure { doSomethingWeWantToMeasure() }
    // ...
    }
 }

 inline fun <T> measure(action: () -> T) {
   val start = SystemClock.elapsedRealtime()
   val result = action()
   val duration = SystemClock.elapsedRealtime() - start
   log(duration)
   return result
 }

Now I can concentrate on reading what the intercept() function’s main intention is without skipping over lines of measurement code. We also benefit from the option of reusing that code in other places where we want to

inline allows you to call a function with a lambda argument within a closure ({ ... }) rather than passing the lambda like measure(myLamda)

When is this useful?

The inline keyword is useful for functions that accept other functions, or lambdas, as arguments.

Without the inline keyword on a function, that function's lambda argument gets converted at compile time to an instance of a Function interface with a single method called invoke(), and the code in the lambda is executed by calling invoke() on that Function instance inside the function body.

With the inline keyword on a function, that compile time conversion never happens. Instead, the body of the inline function gets inserted at its call site and its code is executed without the overhead of creating a function instance.

Hmmm? Example in android -->

Let's say we have a function in an activity router class to start an activity and apply some extras

fun startActivity(context: Context,
              activity: Class<*>,
              applyExtras: (intent: Intent) -> Unit) {
  val intent = Intent(context, activity)
  applyExtras(intent)
  context.startActivity(intent)
  }

This function creates an intent, applies some extras by calling the applyExtras function argument, and starts the activity.

If we look at the compiled bytecode and decompile it to Java, this looks something like:

void startActivity(Context context,
               Class activity,
               Function1 applyExtras) {
  Intent intent = new Intent(context, activity);
  applyExtras.invoke(intent);
  context.startActivity(intent);
  }

Let's say we call this from a click listener in an activity:

override fun onClick(v: View) {
router.startActivity(this, SomeActivity::class.java) { intent ->
intent.putExtra("key1", "value1")
intent.putExtra("key2", 5)
}
 }

The decompiled bytecode for this click listener would then look like something like this:

@Override void onClick(View v) {
router.startActivity(this, SomeActivity.class, new Function1() {
@Override void invoke(Intent intent) {
  intent.putExtra("key1", "value1");
  intent.putExtra("key2", 5);
}
 }
}

A new instance of Function1 gets created every time the click listener is triggered. This works fine, but it's not ideal!

Now let's just add inline to our activity router method:

inline fun startActivity(context: Context,
                     activity: Class<*>,
                     applyExtras: (intent: Intent) -> Unit) {
 val intent = Intent(context, activity)
 applyExtras(intent)
 context.startActivity(intent)
 }

Without changing our click listener code at all, we're now able to avoid the creation of that Function1 instance. The Java equivalent of the click listener code would now look something like:

@Override void onClick(View v) {
Intent intent = new Intent(context, SomeActivity.class);
intent.putExtra("key1", "value1");
intent.putExtra("key2", 5);
context.startActivity(intent);
}

Thats it.. :)

To "inline" a function basically means to copy a function's body and paste it at the function's call site. This happens at compile time.

Use inline for preventing object creation

Lambdas are converted to classes

In Kotlin/JVM, function types (lambdas) are converted to anonymous/regular classes that extend the interface Function. Consider the following function:

fun doSomethingElse(lambda: () -> Unit) {
    println("Doing something else")
    lambda()
}

The function above, after compilation will look like following:

public static final void doSomethingElse(Function0 lambda) {
    System.out.println("Doing something else");
    lambda.invoke();
}

The function type () -> Unit is converted to the interface Function0.

Now let's see what happens when we call this function from some other function:

fun doSomething() {
    println("Before lambda")
    doSomethingElse {
        println("Inside lambda")
    }
    println("After lambda")
}

Problem: objects

The compiler replaces the lambda with an anonymous object of Function type:

public static final void doSomething() {
    System.out.println("Before lambda");
    doSomethingElse(new Function() {
            public final void invoke() {
            System.out.println("Inside lambda");
        }
    });
    System.out.println("After lambda");
}

The problem here is that, if you call this function in a loop thousands of times, thousands of objects will be created and garbage collected. This affects performance.

Solution: inline

By adding the inline keyword before the function, we can tell the compiler to copy that function's code at call-site, without creating the objects:

inline fun doSomethingElse(lambda: () -> Unit) {
    println("Doing something else")
    lambda()
}

This results in the copying of the code of the inline function as well as the code of the lambda() at the call-site:

public static final void doSomething() {
    System.out.println("Before lambda");
    System.out.println("Doing something else");
    System.out.println("Inside lambda");
    System.out.println("After lambda");
}

This doubles the speed of the execution, if you compare with/without inline keyword with a million repetitions in a for loop. So, the functions that take other functions as arguments are faster when they are inlined.

---

Use inline for preventing variable capturing

When you use the local variables inside the lambda, it is called variable capturing(closure):

fun doSomething() {
    val greetings = "Hello"                // Local variable
    doSomethingElse {
        println("$greetings from lambda")  // Variable capture
    }
}

If our doSomethingElse() function here is not inline, the captured variables are passed to the lambda via the constructor while creating the anonymous object that we saw earlier:

public static final void doSomething() {
    String greetings = "Hello";
    doSomethingElse(new Function(greetings) {
            public final void invoke() {
            System.out.println(this.$greetings + " from lambda");
        }
    });
}

If you have many local variables used inside the lambda or calling the lambda in a loop, passing every local variable through the constructor causes the extra memory overhead. Using the inline function in this case helps a lot, since the variable is directly used at the call-site.

So, as you can see from the two examples above, the big chunk of performance benefit of inline functions is achieved when the functions take other functions as arguments. This is when the inline functions are most beneficial and worth using. There is no need to inline other general functions because the JIT compiler already makes them inline under the hood, whenever it feels necessary.

---

Use inline for better control flow

Since non-inline function type is converted to a class, we can't write the return statement inside the lambda:

fun doSomething() {
    doSomethingElse {
        return    // Error: return is not allowed here
    }
}

This is known as non-local return because it's not local to the calling function doSomething(). The reason for not allowing the non-local return is that the return statement exists in another class (in the anonymous class shown previously). Making the doSomethingElse() function inline solves this problem and we are allowed to use non-local returns because then the return statement is copied inside the calling function.

---

Use inline for reified type parameters

While using generics in Kotlin, we can work with the value of type T. But we can't work with the type directly, we get the error Cannot use 'T' as reified type parameter. Use a class instead:

fun <T> doSomething(someValue: T) {
    println("Doing something with value: $someValue")               // OK
    println("Doing something with type: ${T::class.simpleName}")    // Error
}

This is because the type argument that we pass to the function is erased at runtime. So, we cannot possibly know exactly which type we are dealing with.

Using an inline function along with the reified type parameter solves this problem:

inline fun <reified T> doSomething(someValue: T) {
    println("Doing something with value: $someValue")               // OK
    println("Doing something with type: ${T::class.simpleName}")    // OK
}

Inlining causes the actual type argument to be copied in place of T. So, for example, the T::class.simpleName becomes String::class.simpleName, when you call the function like doSomething("Some String"). The reified keyword can only be used with inline functions.

---

Avoid inline when calls are repetitive

Let's say we have the following function that is called repetitively at different abstraction levels:

inline fun doSomething() {
    println("Doing something")
}

First abstraction level

inline fun doSomethingAgain() {
    doSomething()
    doSomething()
}

Results in:

public static final void doSomethingAgain() {
    System.out.println("Doing something");
    System.out.println("Doing something");
}

At first abstraction level, the code grows at: 2¹ = 2 lines.

Second abstraction level

inline fun doSomethingAgainAndAgain() {
    doSomethingAgain()
    doSomethingAgain()
}

Results in:

public static final void doSomethingAgainAndAgain() {
    System.out.println("Doing something");
    System.out.println("Doing something");
    System.out.println("Doing something");
    System.out.println("Doing something");
}

At second abstraction level, the code grows at: 2² = 4 lines.

Third abstraction level

inline fun doSomethingAgainAndAgainAndAgain() {
    doSomethingAgainAndAgain()
    doSomethingAgainAndAgain()
}

Results in:

public static final void doSomethingAgainAndAgainAndAgain() {
    System.out.println("Doing something");
    System.out.println("Doing something");
    System.out.println("Doing something");
    System.out.println("Doing something");
    System.out.println("Doing something");
    System.out.println("Doing something");
    System.out.println("Doing something");
    System.out.println("Doing something");
}

At third abstraction level, the code grows at: 2³ = 8 lines.

Similarly, at the fourth abstraction level, the code grows at 2⁴ = 16 lines and so on.

The number 2 is the number of times the function is called at each abstraction level. As you can see the code grows exponentially not only at the last level but also at every level, so that's 16 + 8 + 4 + 2 lines. I have shown only 2 calls and 3 abstraction levels here to keep it concise but imagine how much code will be generated for more calls and more abstraction levels. This increases the size of your app. This is another reason why you shouldn't inline each and every function in your app.

---

Avoid inline in recursive cycles

Avoid using the inline function for recursive cycles of function calls as shown in the following code:

// Don't use inline for such recursive cycles

inline fun doFirstThing() { doSecondThing() }
inline fun doSecondThing() { doThirdThing() }
inline fun doThirdThing() { doFirstThing() }

This will result in a never ending cycle of the functions copying the code. The compiler gives you an error: The 'yourFunction()' invocation is a part of inline cycle.

---

Can't use inline when hiding implementation

The public inline functions cannot access private functions, so they cannot be used for implementation hiding:

inline fun doSomething() {
    doItPrivately()  // Error
}

private fun doItPrivately() { }

In the inline function shown above, accessing the private function doItPrivately() gives an error: Public-API inline function cannot access non-public API fun.

---

Checking the generated code

Now, about the second part of your question:

but I found that there is no function object created by kotlin for a non-inline function. why?

The Function object is indeed created. To see the created Function object, you need to actually call your lock() function inside the main() function as follows:

fun main() {
    lock { println("Inside the block()") }
}

Generated class

The generated Function class doesn't reflect in the decompiled Java code. You need to directly look into the bytecode. Look for the line starting with:

final class your/package/YourFilenameKt$main$1 extends Lambda implements Function0 { }

This is the class that is generated by the compiler for the function type that is passed to the lock() function. The main$1 is the name of the class that is created for your block() function. Sometimes the class is anonymous as shown in the example in the first section.

Generated object

In the bytecode, look for the line starting with:

GETSTATIC your/package/YourFilenameKt$main$1.INSTANCE

INSTANCE is the object that is created for the class mentioned above. The created object is a singleton, hence the name INSTANCE.

---

That's it! Hope that provides useful insight into inline functions.

The most important case when we use the inline modifier is when we define util-like functions with parameter functions. Collection or string processing (like filter, map or joinToString) or just standalone functions are a perfect example.

This is why the inline modifier is mostly an important optimization for library developers. They should know how it works and what are its improvements and costs. We should use the inline modifier in our projects when we define our own util functions with function type parameters.

If we don’t have function type parameter, reified type parameter, and we don’t need non-local return, then we most likely shouldn’t use the inline modifier. This is why we will have a warning on Android Studio or IDEA IntelliJ.

Also, there is a code size problem. Inlining a large function could dramatically increase the size of the bytecode because it's copied to every call site. In such cases, you can refactor the function and extract code to regular functions.

One simple case where you might want one is when you create a util function that takes in a suspend block. Consider this.

fun timer(block: () -> Unit) {
    // stuff
    block()
    //stuff
}

fun logic() { }

suspend fun asyncLogic() { }

fun main() {
    timer { logic() }

    // This is an error
    timer { asyncLogic() }
}

In this case, our timer won't accept suspend functions. To solve it, you might be tempted to make it suspend as well

suspend fun timer(block: suspend () -> Unit) {
    // stuff
    block()
    // stuff
}

But then it can only be used from coroutines/suspend functions itself. Then you'll end up making an async version and a non-async version of these utils. The problem goes away if you make it inline.

inline fun timer(block: () -> Unit) {
    // stuff
    block()
    // stuff
}

fun main() {
    // timer can be used from anywhere now
    timer { logic() }

    launch {
        timer { asyncLogic() }
    }
}

Here is a kotlin playground with the error state. Make the timer inline to solve it.