title: What is the difference in Swift between 'unowned(safe)' and 'unowned(unsafe)'?
body: <p>Apple's <a href="https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/index.html"><em>Swift Programming Language Guide</em></a> mentions the <em>capture specifiers</em> <code>unowned(safe)</code> and <code>unowned(unsafe)</code>, in addition to <code>weak</code> and <code>unowned</code>. </p>
  
  <p>I (think I) understand the differences between <code>weak</code> and <code>unowned</code>; but what is the difference between <code>unowned(safe)</code> and <code>unowned(unsafe)</code>? The guide doesn't say.</p>
  
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  <p>Please: Don't rely on simply stating an Objective-C equivalent.</p>