Invoke Operator & Operator Overloading in Kotlin

user_5294091 0
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I get to know about the Invoke operator that,

a() is equivalent to a.invoke()

Is there anything more regarding Invoke operator than please explain. Also, I did not get any example of Invoke operator overloading.

Is Invoke operator overloading possible? If possible then can anyone please explain about the Invoke operator overloading with an example? I did not get anything regarding this.

Thanks in advance.

kotlin operator-overloading
5 Answers

Operator Function invoke() Kotlin provides an interesting function called invoke, which is an operator function. Specifying an invoke operator on a class allows it to be called on any instances of the class without a method name.

Let’s see this in action:

class Greeter(val greeting: String) {
    operator fun invoke(name: String) {
        println("$greeting $name")
    }
}

fun main(args: Array<String>) {
    val greeter = Greeter(greeting = "Welcome")
    greeter(name = "Kotlin")
    //this calls the invoke function which takes String as a parameter
}

A few things to note about invoke() here. It:

  • Is an operator function.
  • Can take parameters.
  • Can be overloaded.
  • Is being called on the instance of a Greeter class without method name.
user_189891380

In addition to the other answers:

It's possible to define a class extending an anonymous function.

class SpecialFunction : () -> Unit {}

In such case, the operator invoke is already defined, so it needs to be overriden:

class MyFunction : () -> Unit {
    override fun invoke() { println("Hi Mom") }
}

One more thing about syntax repercussions:

If such "functor" is called right after constructing it, you end up with double parentheses:

MyFunction()()

And, if such functor returns another functor, you may see some obscurities like

MyFunction()()()()()...

perhaps including parameters. This will not surprise anyone coming from the JavaScript world, though.

user_1459890

If you have some Python background,

you can think invoke in Kotlin as __call__ in Python.

By using this, you can "call" your object as if it's a function.

One difference is: you can overload invoke, but there is no official way to overload methods in Python.

user_100275920
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