What does an ampersand (&) mean in the Swift language?

Question

I know about the ampersand as a bit operation but sometimes I see it in front of variable names. What does putting an & in front of variables do?

Answer 1

It means that it is an in-out variable. You can do something directly with that variable. It is passed by address, not as a copy.

For example:

var temp = 10
func add(inout a: Int){
    a++
}
add(inout:&temp)
temp // 11

Answer 2

It works as an inout to make the variable an in-out parameter. In-out means in fact passing value by reference, not by value. And it requires not only to accept value by reference, by also to pass it by reference, so pass it with & - foo(&myVar) instead of just foo(myVar)

As you see you can use that in error handing in Swift where you have to create an error reference and pass it to the function using & the function will populate the error value if an error occur or pass the variable back as it was before

Why do we use it? Sometimes a function already returns other values and just returning another one (like an error) would be confusing, so we pass it as an inout. Other times we want the values to be populated by the function so we don't have to iterate over lots of return values, since the function already did it for us - among other possible uses.

I hope that helps you!

Answer 3

There's another function of the ampersand in the Swift language that hasn't been mentioned yet. Take the following example:

protocol Foo {}
protocol Bar {}

func myMethod(myVar: Foo & Bar) {
    // Do something
}

Here the ampersand syntax is stating that myVar conforms to both the Foo and Bar protocol.

As another use case, consider the following:

func myMethod() -> UIViewController & UITableViewDataSource {
    // Do something
}

Here we're saying that the method returns a class instance (of UIViewController) that conforms to a certain protocol (UITableViewDataSource). This is rendered somewhat obsolete with Swift 5.1's Opaque Types but you may see this syntax in pre-Swift 5.1 code from time to time.

Answer 4

If you put & before a variable in a function, that means this variable is inout variable.

@Icaro already described what it means, I will just give an example to illustrate the difference between inout variables and in variables:

func majec(inout xValue:Int, var yValue:Int) {
    xValue = 100
    yValue = 200
}

var xValue = 33
var yValue = 33
majec(&xValue, yValue: yValue)
xValue //100
yValue //33