SWIFT--文档地址 APPLE官方: https://developer.apple.com/library/prerelease/content/documentation/Swift/Conceptual/Swift_Programming_Language/BasicOperators.html#//apple_ref/doc/uid/TP40014097-CH6-ID60 中文: http://wiki.jikexueyuan.com/project/swift/chapter2/01_The_Basics.html