Operator overloading

Kotlin allows you to provide custom implementations for the predefined set of operators on types. These operators have predefined symbolic representation (like + or *) and precedence. To implement an operator, provide a member function or an extension function with a specific name for the corresponding type. This type becomes the left-hand side type for binary operations and the argument type for the unary ones.

To overload an operator, mark the corresponding function with the operator modifier:

interface IndexedContainer {
    operator fun get(index: Int)
}

When overriding your operator overloads, you can omit operator:

class OrdersList: IndexedContainer {
    override fun get(index: Int) { /*...*/ }   
}

Unary operations

Unary prefix operators

Expression	Translated to
`+a`	`a.unaryPlus()`
`-a`	`a.unaryMinus()`
`!a`	`a.not()`

This table says that when the compiler processes, for example, an expression +a, it performs the following steps:

Determines the type of a, let it be T.
Looks up a function unaryPlus() with the operator modifier and no parameters for the receiver T, that means a member function or an extension function.
If the function is absent or ambiguous, it is a compilation error.
If the function is present and its return type is R, the expression +a has type R.

As an example, here's how you can overload the unary minus operator:

data class Point(val x: Int, val y: Int)

operator fun Point.unaryMinus() = Point(-x, -y)

val point = Point(10, 20)

fun main() {
   println(-point)  // prints "Point(x=-10, y=-20)"
}

Increments and decrements

Expression	Translated to
`a++`	`a.inc()` + see below
`a--`	`a.dec()` + see below

The inc() and dec() functions must return a value, which will be assigned to the variable on which the ++ or -- operation was used. They shouldn't mutate the object on which the inc or dec was invoked.

The compiler performs the following steps for resolution of an operator in the postfix form, for example a++:

Determines the type of a, let it be T.
Looks up a function inc() with the operator modifier and no parameters, applicable to the receiver of type T.
Checks that the return type of the function is a subtype of T.

The effect of computing the expression is:

Store the initial value of a to a temporary storage a0.
Assign the result of a0.inc() to a.
Return a0 as the result of the expression.

For a-- the steps are completely analogous.

For the prefix forms ++a and --a resolution works the same way, and the effect is:

Assign the result of a.inc() to a.
Return the new value of a as a result of the expression.

Binary operations

Arithmetic operators

Expression	Translated to
`a + b`	`a.plus(b)`
`a - b`	`a.minus(b)`
`a * b`	`a.times(b)`
`a / b`	`a.div(b)`
`a % b`	`a.rem(b)`
`a..b`	`a.rangeTo(b)`
`a..<b`	`a.rangeUntil(b)`

For the operations in this table, the compiler just resolves the expression in the Translated to column.

Below is an example Counter class that starts at a given value and can be incremented using the overloaded + operator:

data class Counter(val dayIndex: Int) {
    operator fun plus(increment: Int): Counter {
        return Counter(dayIndex + increment)
    }
}

in operator

Expression	Translated to
`a in b`	`b.contains(a)`
`a !in b`	`!b.contains(a)`

For in and !in the procedure is the same, but the order of arguments is reversed.

Indexed access operator

Expression	Translated to
`a[i]`	`a.get(i)`
`a[i, j]`	`a.get(i, j)`
`a[i_1, ..., i_n]`	`a.get(i_1, ..., i_n)`
`a[i] = b`	`a.set(i, b)`
`a[i, j] = b`	`a.set(i, j, b)`
`a[i_1, ..., i_n] = b`	`a.set(i_1, ..., i_n, b)`

Square brackets are translated to calls to get and set with appropriate numbers of arguments.

invoke operator

Expression	Translated to
`a()`	`a.invoke()`
`a(i)`	`a.invoke(i)`
`a(i, j)`	`a.invoke(i, j)`
`a(i_1, ..., i_n)`	`a.invoke(i_1, ..., i_n)`

Parentheses are translated to calls to invoke with appropriate number of arguments.

Augmented assignments

Expression	Translated to
`a += b`	`a.plusAssign(b)`
`a -= b`	`a.minusAssign(b)`
`a *= b`	`a.timesAssign(b)`
`a /= b`	`a.divAssign(b)`
`a %= b`	`a.remAssign(b)`

For the assignment operations, for example a += b, the compiler performs the following steps:

If the function from the right column is available:
- If the corresponding binary function (that means plus() for plusAssign()) is available too, a is a mutable variable, and the return type of plus is a subtype of the type of a, report an error (ambiguity).
- Make sure its return type is Unit, and report an error otherwise.
- Generate code for a.plusAssign(b).
Otherwise, try to generate code for a = a + b (this includes a type check: the type of a + b must be a subtype of a).

Equality and inequality operators

Expression	Translated to
`a == b`	`a?.equals(b) ?: (b === null)`
`a != b`	`!(a?.equals(b) ?: (b === null))`

These operators only work with the function equals(other: Any?): Boolean, which can be overridden to provide custom equality check implementation. Any other function with the same name (like equals(other: Foo)) will not be called.

The == operation is special: it is translated to a complex expression that screens for null's. null == null is always true, and x == null for a non-null x is always false and won't invoke x.equals().

Comparison operators

Expression	Translated to
`a > b`	`a.compareTo(b) > 0`
`a < b`	`a.compareTo(b) < 0`
`a >= b`	`a.compareTo(b) >= 0`
`a <= b`	`a.compareTo(b) <= 0`

All comparisons are translated into calls to compareTo, that is required to return Int.

Property delegation operators

provideDelegate, getValue and setValue operator functions are described in Delegated properties.

Infix calls for named functions

You can simulate custom infix operations by using infix function calls.

Last modified: 25 September 2024