Kotlin is a programming language that provides a lot of powerful features to make code concise and expressive. One of these features is inline functions – a technique to improve the performance of higher-order functions. In this topic, we'll explore the concept of inline functions, their benefits, and their use in Kotlin.
What are inline functions?
An inline function is a function that is expanded at the call site instead of being executed as a separate function call. In other words, the body of an inline function is copied and pasted directly into the calling code, just like a macro in C/C++ or a template in C++.
By default, all functions in Kotlin are non-inline, meaning that they are executed as separate function calls. However, when we mark a function as inline, the compiler replaces the function call with the function's body. This reduces the overhead of the function call as well as the creation of unnecessary objects.
To mark a function as inline, we use the inline keyword before the function declaration. Here's the syntax:
inline fun functionName(parameters): returnType {
// function body
}Let's look at an example and figure out what is happening:
inline fun measureTimeMillis(block: () -> Unit): Long {
val startTime = System.currentTimeMillis()
block()
return System.currentTimeMillis() - startTime
}
fun main() {
val time = measureTimeMillis {
// do some time-consuming operation
Thread.sleep(1000)
}
println("Time taken: $time milliseconds")
}In this example, we define an inline function called measureTimeMillis that takes a lambda expression as a parameter. The lambda expression is expected to perform some time-consuming operation, which we want to measure. Inside the measureTimeMillis function, we first record the start time using System.currentTimeMillis(), then call the lambda expression, and finally subtract the start time from the current time to get the total time taken.
By marking the measureTimeMillis function as inline, the Kotlin compiler will replace any calls to this function with the actual code inside the function body at compile time. This means that there will be no function call overhead at runtime, which can be beneficial for performance-critical code.
In the main function, we call the measureTimeMillis function and, using Thread.sleep(1000), pass a lambda expression that sleeps for one second. The time taken for this operation is returned by measureTimeMillis and stored in the time variable, which is then printed to the console.
In Kotlin, functions are stored in memory in the same way as any other object. When a function is defined in code, it is compiled into bytecode and can then be loaded into memory during program execution.
When the function is called, its code is loaded into memory, and the processor begins to execute this code. Like other objects, functions are stored in the heap, a memory area allocated for storing objects during program execution. When you create a function in a heap, enough memory is allocated to store it, including the function code and any variables or objects needed to run it.
It is important to understand that each time a function is called, a new instance of the function is created in memory. This means that if a function is called repeatedly, many instances of that function will be created in memory.
Take a look at the code below:
fun main() {
val width = 10
val height = 20
println(calculateAreaWithoutInline(width, height))
println(calculateAreaWithoutInline(width, height))
println(calculateAreaWithoutInline(width, height))
println(calculateAreaWithoutInline(width, height))
println(calculateAreaWithoutInline(width, height))
println(calculateAreaWithInline(width, height))
println(calculateAreaWithInline(width, height))
println(calculateAreaWithInline(width, height))
println(calculateAreaWithInline(width, height))
println(calculateAreaWithInline(width, height))
}
fun calculateAreaWithoutInline(width: Int, height: Int): Int {
return width * height
}
inline fun calculateAreaWithInline(width: Int, height: Int): Int = width * heightNow let's consider how the compiler sees this code:
fun main() {
val width = 10
val height = 20
println(calculateAreaWithoutInline(width, height)) // A new function object will be created here
println(calculateAreaWithoutInline(width, height)) // and here
println(calculateAreaWithoutInline(width, height)) // and here
println(calculateAreaWithoutInline(width, height)) // and here
println(calculateAreaWithoutInline(width, height)) // and here
println(width * height) // The compiler copies its body to the place where the function is called
println(width * height) // without creating a new function object
println(width * height) // also here
println(width * height) // also here
println(width * height) // also here
}
fun calculateAreaWithoutInline(width: Int, height: Int): Int {
return width * height
}
inline fun calculateAreaWithInline(width: Int, height: Int): Int = width * heightOverall, inline functions can be a powerful tool in Kotlin for optimizing performance-critical code by eliminating function call overhead. However, they should be used with care, as inlining too much code can lead to an increased binary size and longer compile time.
Benefits of inline functions
The main benefits of inline functions are:
Improved performance: since the code of an inline function is inserted directly into the calling code, there is no need to create an additional function object, which reduces the overhead of function calls.
Reduction of memory usage: the inline function avoids the creation of a function object and its closure object, which leads to a smaller memory footprint.
Higher-order function optimization: inlining functions allows us to use higher-order functions without the overhead of function calls, which makes the code faster and more efficient.
When do we use inline functions?
Inline functions are helpful when we need to use higher-order functions, such as lambda expressions, and when we want to avoid the overhead of function calls. Inline functions should be used in the following scenarios:
Functions that are used as arguments to other functions: inlining a function that is used as an argument to another function will eliminate the overhead of function calls and improve the performance of the code.
Functions that are used frequently: if a function is called frequently, marking it as
inlinecan significantly improve the performance of the code by reducing the overhead of function calls.Functions that return lambda expressions: if a function returns a lambda expression, marking it as
inlinecan help reduce the overhead of function calls when the lambda expression is called.Small functions: inlining small functions can help reduce the overhead of function calls and improve the performance of the code.
Inline functions also have some limitations, which we need to keep in mind:
Code duplication: inlining functions can lead to code duplication, which can increase the size of the compiled code.
Large functions: inlining large functions can lead to an increase in code size, as the entire function body is copied and pasted into the calling code.
Recursive functions: inline functions cannot be recursive, as inlining a recursive function can lead to an infinite loop.
Reified type parameters
In the body of a universal function, you cannot access type T because it is available only at compile time but is erased at runtime. So, if you want to use a generic type as a regular class in the body of a function, you need to explicitly pass the class as a parameter, like this:
fun <T> myFun(c: Class<T>)Reified type parameters are another feature of Kotlin that can be used in conjunction with inline functions to provide additional type information at runtime. Here's how we can use it:
inline fun <reified T> myFun()reified is a keyword in Kotlin that is used to mark a type parameter as being available at runtime. In other words, reified type parameters are those that are not erased by the compiler but, instead, retain their type information at runtime. By marking a type parameter as reified, it is possible to use it as a regular class object, with all its type information available at runtime. To use reified type parameters in an inline function, the type parameter must be marked with the reified keyword. Here is an example of an inline function that uses a reified type parameter:
inline fun <reified T> printType() {
println(T::class.qualifiedName)
}In the above example, the printType() function is marked as inline and has a type parameter T, which is marked as reified. The function prints the qualified name of the class represented by the reified type parameter. The main benefit of using reified type parameters in inline functions is that it allows for more flexible and powerful type checking and casting at runtime. This can be particularly useful in cases where the type information of generic type parameters is needed at runtime. Additionally, reified type parameters can help reduce the amount of boilerplate code needed in certain situations, making the code more concise and easier to read.
Generics type checks and casts
Now that we've explored inline functions and reified type parameters, we can leverage these features to perform type checks and casts with generics at runtime — something normally impossible due to type erasure. When working with generics, we may need to check whether an object is an instance of a specific type parameter or cast it to a type parameter.
To check whether an object is an instance of a specific type parameter, we can use the is operator with the type parameter in angle brackets. For example:
fun <T> exampleFunction(obj: Any) {
if (obj is T) {
// obj is an instance of type parameter T
} else {
// obj is not an instance of type parameter T
}
}Similarly, we can cast an object to a type parameter using the as operator with the type parameter in angle brackets. However, if the object is not an instance of the type parameter, ClassCastException will be thrown. To avoid this, we can use the safe cast operator as?, which returns null if the cast is not possible.
fun <T> exampleFunction(obj: Any) {
val tObj: T? = obj as? T
if (tObj != null) {
// obj can be safely cast to type parameter T
} else {
// obj cannot be cast to type parameter T
}
}It's important to note that type erasure occurs with generics in Kotlin, meaning that the actual type of a generic object is not known at runtime. Therefore, certain operations, like creating a new instance of a type parameter or checking if a type parameter is a subtype of another class, are not possible.
In summary, type checks and casts with generics in Kotlin can be done using the is and as operators with the type parameter in angle brackets, and the safe cast operator as? can be used to avoid ClassCastExceptions. However, certain operations may not be possible due to type erasure.
Conclusion
Inline functions are a powerful tool in Kotlin that allows us to optimize the performance of higher-order functions. They are useful in scenarios where we want to avoid the overhead of function calls and improve the performance of the code. However, they also have some limitations, such as code duplication and restrictions on the use of recursive functions. Therefore, we should use them judiciously and only in scenarios where they provide a measurable benefit to the performance of the code. And also remember that reified type parameters are a powerful feature of Kotlin that can be used in conjunction with inline functions to provide additional type information at runtime. This can lead to more flexible and powerful type checking and casting at runtime, making code more concise and easier to read.