# Operator overloading

Kotlin allows us to provide implementations for a predefined set of operators on our types. These operators have fixed symbolic representation
(like `+`

or `*`

) and fixed precedence. To implement an operator, we provide a member function
or an extension function with a fixed name, for the corresponding type, i.e. left-hand side type for binary operations and argument type for unary ones.
Functions that overload operators need to be marked with the `operator`

modifier.

Further we describe the conventions that regulate operator overloading for different operators.

## 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`

, i.e. 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`

.

*Note* that these operations, as well as all the others, are optimized for Basic types and do not introduce overhead of function calls for them.

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)
println(-point) // prints "(-10, -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, e.g. `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
`a.inc()`

to`a`

, - Return
`a0`

as a 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.mod(b)` (deprecated) |

`a..b ` |
`a.rangeTo(b)` |

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

Note that the `rem`

operator is supported since Kotlin 1.1. Kotlin 1.0 uses the `mod`

operator, which is deprecated
in Kotlin 1.1.

#### Example

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.modAssign(b)` |

For the assignment operations, e.g. `a += b`

, the compiler performs the following steps:

- If the function from the right column is available
- If the corresponding binary function (i.e.
`plus()`

for`plusAssign()`

) is available too, report error (ambiguity). - Make sure its return type is
`Unit`

, and report an error otherwise. - Generate code for
`a.plusAssign(b)`

- If the corresponding binary function (i.e.
- 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`

).

*Note*: assignments are *NOT* expressions in Kotlin.

### Equality and inequality operators

Expression | Translated to |
---|---|

`a == b` |
`a?.equals(b) ?: (b === null)` |

`a != b` |
`!(a?.equals(b) ?: (b === null))` |

*Note*: `===`

and `!==`

(identity checks) are not overloadable, so no conventions exist for them

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`

.

## Infix calls for named functions

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