https://developer.apple.com/library/prerelease/content/documentation/Swift/Conceptual/Swift_Programming_Language/AutomaticReferenceCounting.html#//apple_ref/doc/uid/TP40014097-CH20-ID48https://developer.apple.com/library/prerelease/
阅读背景：
Swift3.0学习笔记-Automatic Reference Counting
来源：互联网
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