[Effective ObjectiveC 2.0 Note] 1.The memory for objects is always allocated in heap space and never on the stack. 2.对于[NSArray arrayWithObjects:obj1,obj2,obj3,nil].当使用方法时,如果obj2为空,则只有obj1被插入.录使用Literals时,@[objc3,obj2,obj3],如果objc2为空,会抛出异常.dictionary…