来自SwiftProgrammingLanguage文档:ImplicitlyUnwrappedOptionalTypeTheSwiftlanguagedefinesthepostfix!assyntacticsugarforthenamedtypeImplicitlyUnwrappedOptional,whichisdefinedintheSwiftstandardlibrary.Inotherwords,thefollowingtwodeclarationsareequivalent:varimplicitlyUnwrappedString:String!varimplicitly