Have you ever had a chance to see code similar to the one below?

infix operator +++ {associativity left precedence 100 }
func +++ (letf: X, right: X) {
	return ...

and you haven’t really known what all these words mean?

Let’s decode all keywords and make some simple examples to explain everything!

But before we do that …

Above code is used to define new custom operators in Swift. These operators can have no existing meaning defined in Swift, like in example +++. New operators are declared at the global level, so they are visible from everywhere in our code. After this short introduction we can start. so! what does infix mean?


Infixed operators are for example: plus, minus, multiplication, etc… They just take a left and right hand argument:

  • These are infixed operators: 2 + 2, 2 * 2, 2 / 2
  • These aren’t: 2 , ++ 2

Easy :)


Keyword operator has to always appear before definition of our custom set of characters.

+++, —, |||, ????, …., and much more other options

The best explanation about which characters we can use to create our custom operator you can find here

And finally two magic parameters:


This parameter says how operators of the same precedence are grouped in the absence of parentheses: from left or from right. If operator is left associative, then we go from the left to the right:

(a o b) o c

And if it is right associative, then:

a o (b o c)

Example with left assocativate operator

var x = 12 / 6 / 2 

Becesuse / is left associativate it is evaluated as following: :

var x  = (12  / 6 ) / 2     // 1

If it were right associative then it would be evaluated as:

var x = 12  / ( 6  / 2 )     // 4 

Example with right assocativate operator

var x = 5
x += 2 

It means that at the beginning 5 will be increased by two, and then value will be assigned to x, so operation will be evaluated from right to left:

x = (5 + 2) 


Operator precedence gives some operators higher priority than others and these operators are applied first. For example operator * is applied before + , because it has higher precedence.

That’s why 2 + 2 * 2 = 6 not 8 ☺ So simple! :)

Default values

parameter possible Values default value
associativity left, right, and none none
precedence 0-255 100

To feel better these parameters, below are some examples of existing operators in Swift with their precedence and associativity values:

Operator precedence associativity
+ 140 left
* 150 left
/ 150 left
+= 90 right
== 90 none

Precedence and associativity can be only defined for infix operators. There is no reason to use their for prefix and postfix operators, so we can simply write:

prefix operator +++ {}


In this post I presented only the basic clue of custom operators in swift but I hope that now you feel stronger in infix operators subject and you are ready to create your own operators:) If you want to expand your operators knowledge, then definitely visit the following websites:

Have a nice day!:)