[C/C++ basic] Preprocessing #if defined(SHAWN)

It's lot of usage same as #ifdef Like #if defined(...) When using #if defined(...) we can used logic AND/OR for preprocessing.

Sample 1:

#ifdef ABC
// do something
#elif DEF
// do something
#else
// do something else
#endif

Sample 1 above can be simply recoding as:

#if defined(ABC) || defined(DEF)
//do something
#else
// do something else
#endif