苹果ProgrammingwithObjective-C文档指出:Youshouldalwaysaccesstheinstancevariablesdirectlyfromwithinaninitializationmethodbecauseatthetimeapropertyisset,therestoftheobjectmaynotyetbecompletelyinitialized.Evenifyoudon’tprovidecustomaccessormethodsorknowofanysideeffectsfromwithinyourownclass,afuturesubcla