// #define FLAG


#ifdef FLAG

  You will see this line if the symbol FLAG is defined

#else

  You will see this line if the symbol FLAG is not defined

#endif


You will always see this line, because it is outside any ifdef/ifndef



#ifndef FLAG

  Another line. You will see this line if the symbol FLAG is not defined

#else

  Another line. You will see this line if the symbol FLAG is defined

#endif