what's the purpose of double question mark in swift

Question

I have seen such function:

public func highlightValues(highs: [ChartHighlight]?)
{
    // set the indices to highlight
    _indicesToHightlight = highs ?? [ChartHighlight]();

    // redraw the chart
    setNeedsDisplay();
}

What's the purpose of ?? here? I searched, but it seems searching ?? is hard to find a proper answer.

score 176 · Accepted Answer · answered Jun 15 '15 at 03:55

176

It is called nil coalescing operator. If highs is not nil than it is unwrapped and the value returned. If it is nil then [ChartHighlight]() returned. It is a way to give a default value when an optional is nil.

answered Jun 15 '15 at 03:55

mustafa

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The nil-coalescing operator a ?? b is a shortcut for a != nil ? a! : b – Marcelo Gracietti Nov 14 '16 at 00:10
3

This is very handy. In Javascript, you can use OR operator to achieve the same, like so: var thisIsNeverNull = thisStringMightBeNull || "" – Fredrik Johansson Feb 06 '18 at 14:21
Its good to give guarantee to a object will not be nil with using ??. Thanks for answer. – eemrah Feb 14 '18 at 07:25
For clarity: If `a = false`, `a ?? b` will return `false`. – Hari Honor Oct 07 '21 at 13:11

what's the purpose of double question mark in swift

1 Answers1