Monday, 9 February 2015

Nil Coalescing Operator Swift

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.


NOTE

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"

The userDefinedColorName variable is defined as an optional String, with a default value of nil. Because userDefinedColorName is of an optional type, you can use the nil coalescing operator to consider its value. In the example above, the operator is used to determine an initial value for a String variable called colorNameToUse. Because userDefinedColorName is nil, the expression userDefinedColorName ?? defaultColorName returns the value of defaultColorName, or "red".

If you assign a non-
nil value to userDefinedColorName and perform the nil coalescing operator check again, the value wrapped inside userDefinedColorName is used instead of the default:

userDefinedColorName = "green"
 colorNameToUse = userDefinedColorName ?? defaultColorName

// userDefinedColorName is nil, so colorNameToUse is set to the default of "green" 

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