#ifdef预编译相关用法主要有：
(1)#ifdef XXX
executing the corresponding xxx code
#endif
(2)#ifdef XXX
executing the corresponding xxx code
#else
executing the other code
(3)#ifdef XXX
executing the corresponding xxx code
#elif defined YYY
executing the corresponding yyy code
#else
executing the other code
#endif
注意最后要使用#endif结束