从此参考：
When a new object is created,memory for it is allocated,and its
instance variables are initialized. First among the object’s variables
is a pointer to its class structure. This pointer,called isa,gives
the object access to its class and,through the class,to all the
classes it inherits from.
从我可以理解的this和this,它允许一个人使用内省但是以务实的方式(iOS开发),程序员可以从知道/理解这个特殊的指针获得什么？
