NSSet持有对其对象的强引用，所以ObjectiveC的解决方案是使用NSHashTable.weakObjectsHashTable()alaNSHashTable*mySet=[NSHashTableweakObjectsHashTable];[mySetaddObject:anyOldObject];[mySetcount];//returns1[mySetcontainsObject:anyOldObject];//returnstrue在swift中，但这似乎不起作用varmySet=NSHashTable.weakObjectsHashTable()mySet.addOb