title: what's the purpose of double question mark in swift
body: <p>I have seen such function:</p>
  
  <pre><code>public func highlightValues(highs: [ChartHighlight]?)
  {
      // set the indices to highlight
      _indicesToHightlight = highs ?? [ChartHighlight]();
  
      // redraw the chart
      setNeedsDisplay();
  }
  </code></pre>
  
  <p>What's the purpose of <code>??</code> here? I searched, but it seems searching <code>??</code> is hard to find a proper answer.</p>