What is the meaning of “??” in swift? [duplicate]

问题

I came across a syntax in Swift where double question marks are used ("??").

For instance, let val = something["something"] as? String ?? nil

What exactly does this mean? What are some use cases for this syntax?

回答1:

Nil-Coalescing Operator

It's kind of a short form of this. (Means you can assign default value nil or any other value if something["something"] is nil or optional)

let val = (something["something"] as? String) != nil ? (something["something"] as! String) : "default value"

The nil-coalescing operator (a ?? b) unwraps an optional a if it contains a value, or returns a default value b if a is nil. The expression a is always of an optional type. The expression b must match the type that is stored inside a.

The nil-coalescing operator is shorthand for the code below:

a != nil ? a! : b The code above uses the ternary conditional operator and forced unwrapping (a!) to access the value wrapped inside a when a is not nil, and to return b otherwise. The nil-coalescing operator provides a more elegant way to encapsulate this conditional checking and unwrapping in a concise and readable form.

If the value of a is non-nil, the value of b is not evaluated. This is known as short-circuit evaluation.

The example below uses the nil-coalescing operator to choose between a default color name and an optional user-defined color name:

let defaultColorName = "red"
var userDefinedColorName: String?   // defaults to nil

var colorNameToUse = userDefinedColorName ?? defaultColorName
// userDefinedColorName is nil, so colorNameToUse is set to the default of "red"

See section Nil-Coalescing Operator here

回答2:

This operator is generally used to provide a default value when an assigned expression or variable produces an optional result. for ex:

let i: Int? = 5
let j: Int? = nil

let value1 = i ?? 9 //value1 will be 5 non-optional
let value2 = j ?? 9 //value2 will be 9 non-optional

The example you've provided although syntactically correct is redundant, as omitting the ?? nil would have no result on the outcome.

回答3:

This is Nil-Coalescing Operator in Swift.

Nil-Coalescing Operator

The nil-coalescing operator (a ?? b) unwraps an optional a if it contains a value, or returns a default value b if a is nil. The expression a is always of an optional type. The expression b must match the type that is stored inside a.

But in your example you already using optional binding , so no need of using ?? with optional, if it fails condition then it automatically assign nil.

let val = something["something"] as? String ?? nil

Here, something["something"] as? String, already assign nil, if something["something"] fails to deliver String. So no need of this operator here.

来源：https://stackoverflow.com/questions/47362914/what-is-the-meaning-of-in-swift

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