What does the ?? operator mean in Swift

What does the ?? operator mean in Swift

?? is a nil-coalescing operator. It is a binary operator^[1] (a ?? b) that returns the result of its left-most operand if it is not nil and returns the right-most operand otherwise.

The nil-coalescing operator (a ?? b) is shorthand for the code below:

a != nil ? a! : b

If a is not nil, returns a, otherwise return b. The expression a is always of an optional type. The expression b must match the type that’s stored inside a.

Here is a long version for better understanding.

func nilCoalescing(a: String?, b: String) -> String {
    if a != nil {
        return a!
    } else {
        return b
    }
}

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What is the purpose of nil-coalescing operator

Nil-coalescing operator use for defines a default value where a more specific value is an optional type.

In the following example, we let users set a specific string value for a cell's title, but we set it to default if they decided not to do that.

// 1
func configureCell(title: String?) {
    // 2
    titleLabel.text = title ?? "Default title"
}

1 A specific value of type optional string (String?).
2 If a passing title is nil, we use the default value Default title.

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Conclusion

Nil-coalescing isn't exclusive to Swift. It is a common way to define a default value for langauges^[2] that has a concept of null. So if you have a code that requires a default value, don't hesitate to use nil-coalescing. It might look weird at first, but it conveys a clearer message of what you are trying to do than an if-else statement.