Shorthand for assignment if nil in Swift?

Question

if x == nil {
    x = y
}

I know the statement above can be rewritten as:

x = x ?? y

But x ??= y is not accepted by compiler. Is there a shorthand not to repeat x?

Answer 1

Check this out, put the following in global code space(In your Extensions.swift maybe). This creates a custom operator you can use throughout your entire project.

Swift 2

infix operator ??= {}
func ??= <T>(inout left: T?, right: T) {
    left = left ?? right
}

Swift 3

infix operator ??=
func ??= <T>(left: inout T?, right: T) {
    left = left ?? right
}

Usage:

var foo: String? = nil
var bar = "Bar"
foo ??= bar
print("Foo: \(bar)")

Hope this helps :)

Answer 2

Laffen’s answer is great. However, it is not exactly equivalent to x = x ?? y as the right hand side is always evaluated with their definition of ??= as opposed to the standard ?? operator.

If you want to have this behavior too, @autoclosure is here to help:

infix operator ??=
func ??=<T>(left: inout T?, right: @autoclosure () -> T) {
    left = left ?? right()
}

Answer 3

This code works in playground:

var x: Int? = nil
let y = 5
x = x ?? y // x becomes 5

and

var x: Int? = 6
let y = 5
x = x ?? y // x stays 6 

By the way here are some variants of checking for nil:

if x != nil {
    // classic
}

if x != .None {
    // nil in Swift is enum with value .None
}

if let _x = x {
    // if you need it's value
}
if let _ = x {
    // if you don't need it's value
}

UPD: code for project - copy and run it:

var x: Int? = nil
let y = 5
x = x ?? y
print (x)
x = 7
x = x ?? y
print (x)