说明：  C++的四个类型转换运算符已经有很久了,但一直没有弄清楚它们的用法,今天看到一本书上的解释,才大致地的了解了其具体的用法.
(The four types of C++ conversion operator has been a long time, but has not been clear about their usage, see today' s interpretation of a book before or less to the understanding of their specific usage.)