NEW: Master Swift design patterns with my latest book! >>

How to use operator overloading

Written by Paul Hudson    @twostraws

Operator overloading is the practice of adding new operators and modifying existing ones to do different things. Operators are those little symbols like +, *, and /, and Swift uses them in a variety of ways depending on context – a string plus another string equals a combined string, for example, whereas an integer plus another integer equals a summed integer.

To create your own operator you need to tell Swift whether it should be prefix (before its operand; the values used with it), postfix (after its operand), or infix. The most common is infix: +, -, *, and more are all infix.

To create a new operator, try adding this to a playground:

infix operator **

That’s the exponentiation operator, designed to raise one number to the power of another. Normally we’d use the pow() function for that job, but with operator overloading we can make ** work instead.

Now you need to tell Swift what to do when it sees that operator. For example, when we write something like 2 ** 4 what does that mean?

What Swift wants is a function called **, the name of our operator, where the left-hand side is one type and the right-hand side is another type. Which type is down to us, but ** is normally used with a Double on either side, so we’re going to write a function that accepts two doubles and returns a double:

func **(lhs: Double, rhs: Double) -> Double {
    return pow(lhs, rhs)
}

As you can see, the function itself is a cinch thanks to pow() – we literally just pass on the numbers. Now this code should work in your playground:

let result = 2 ** 4

For more advanced uses, you also need to specify associativity and a precedence group, but what we have is fine to start with.

Available from iOS 8.0 – learn more in my book Pro Swift

Did this solution work for you? Please pass it on!

Other people are reading…

About the Swift Knowledge Base

This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.

Go from iOS to macOS the easy way!

If you like Hacking with Swift, you'll love Hacking with macOS – learn to build macOS apps today, using 18 real-world projects!

Click here to visit the Hacking with Swift store >>