About_Define.cpp

#include <iostream>
#include <stdio.h>

#define FOO_Def
#define FOO_Def2 5

int main(int argc, const char * argv[])
{


#ifdef FOO_Def
    printf ("FOOO\n");
#endif
  // it will print out
#ifdef FOO_Def2
    printf ("FOOO\n");
#endif
  // it will print out  
    
    //if (0==FOO_Def) Compiler will error, because FOO_Def don't have any value. 
    if (0==FOO_Def2)  //Need define value for value check.
        printf ("FOOO\n");
    return 0;
}