swift2 - Maintain value semantics in Swift struct, which contains object reference -

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i have swift struct contains object internal storage. how can make sure struct has value semantics?

public struct times {     private let times = nsmutableindexset()      mutating func addtimerange(opentime: int, closetime: int) {         self.times.addindexesinrange(nsrange(location: opentime, length: closetime - opentime))     } }

store nsindexset instead of nsmutableindexset. why immutable superclass exists. 

public struct times {     private var times = nsindexset()     mutating func addtimerange(opentime: int, closetime: int) {         let t = nsmutableindexset(indexset:self.times)         t.addindexesinrange(nsrange(location: opentime, length: closetime - opentime))         self.times = nsindexset(indexset:t)     } }

if class instead of struct, cause last step performed automatically declaring times @nscopying , using simple assignment:

public class times {     @nscopying private var times = nsindexset()     func addtimerange(opentime: int, closetime: int) {         let t = nsmutableindexset(indexset:self.times)         t.addindexesinrange(nsrange(location: opentime, length: closetime - opentime))         self.times = t // ensure immutable copy     } }

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