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How to use operator overloading

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

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