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Nil-Coalescing Operator

Swift

Nil-Coalescing Operator

Don’t be confused by the “??”—we’re not asking a question here. Today, we’re going to study the ?? operator.

This is a super common operator that many developers use without knowing its official name. But even a mere operator deserves to be called by its true name so that it can “bloom” into something useful (as a literature major might say).

1. What is the Nil-Coalescing Operator?

When you use this operator, you no longer need to check whether an optional type contains a value before extracting it.

Here’s the basic syntax:

let result = optionalValue ?? defaultValue

Doesn’t make much sense without an example, right? Let’s take a look at how it works.

Suppose we have an optional string called name:

var name: String? = nil

Now, we want to print:

  • hello, \(name) if name contains a value
  • hello, what's your name? if name is nil

With optional binding, you would write something like this:

if let name = name {
    print("hello, \(name)")
} else {
    print("hello, what's your name?")
}

But with Nil-Coalescing, it simplifies to:

print("hello, \(name ?? "what's your name?")")

One line! So simple, right?

How Does It Work?

① If name contains a value:

let message = "hello, \(name ?? "what's your name?")"

Will output:

hello, John

② If name is nil, the expression evaluates to the default value:

let message = "hello, \(name ?? "what's your name?")"

Will output:

hello, what's your name?

The right-hand side operand ("what's your name?") is used as a default value in case name is nil.

Important Notes:

The optional type on the left-hand side and the non-optional type on the right-hand side must be of the same type, excluding the optionality.

Examples:

  • Optional String ?? Non-Optional String (Valid ✅)
  • Optional String ?? Non-Optional Int (Invalid ❌)

It’s a bit like a ternary operator, right? Since it’s so easy, there’s no need for further explanation!

That’s it for the Nil-Coalescing Operator! Keep it simple and use this operator whenever you want to safely provide a default value for an optional type.

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