let john = Person()
john.residence = Residence()
let johnsAddress = Address()
johnsAddress.buildingName = "The Larches"
johnsAddress.street = "Laurel Street"
john.residence!.address = johnsAddress
let john = Person()
john.residence = Residence(
阅读背景：
为什么在这个例子中使用force unwrapping？
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