Question

I’ve write something like below:

#define ABC20
//#define   ABC10
//#define   ABC00

#ifdef ABC20
    #define  SYSTEMNAME                       "ABC2"
#elif ABC10
    #define  SYSTEMNAME                       "ABC1"
#elif ABC00
    #define  SYSTEMNAME                       "ABC0"
#else
    #define  SYSTEMNAME                       "UNKNOWN"
#endif

And it can work.

But if I change to below:

//#define ABC20
#define   ABC10
//#define   ABC00

#ifdef ABC20
    #define  SYSTEMNAME                       "ABC2"
#elif ABC10
    #define  SYSTEMNAME                       "ABC1"
#elif ABC00
    #define  SYSTEMNAME                       "ABC0"
#else
    #define  SYSTEMNAME                       "UNKNOWN"
#endif

When I compiled, it got error saying ABC10 is not declared. Do you guys know why?

Answer 1

Use defined test:

#ifdef ABC20
  #define  SYSTEMNAME                       "ABC2"
#elif defined ABC10
  #define  SYSTEMNAME                       "ABC1"
#elif defined ABC00
  #define  SYSTEMNAME                       "ABC0"
#else
  #define  SYSTEMNAME                       "UNKNOWN"
#endif

otherwise there is no condition for elif to check because after pre-processing you are left with:   

#ifdef 
   #define  SYSTEMNAME                       "ABC2"
#elif 
  #define  SYSTEMNAME                       "ABC1"
#elif 
  #define  SYSTEMNAME                       "ABC0"
#else
  #define  SYSTEMNAME                       "UNKNOWN"
#endif

but putting in the defined replaces the test with the result so you get:

#if   0
  #define  SYSTEMNAME                       "ABC2"
#elif 1
  #define  SYSTEMNAME                       "ABC1"
#elif 0
  #define  SYSTEMNAME                       "ABC0"
#else
  #define  SYSTEMNAME                       "UNKNOWN"
#endif

Alternatively, you can define on flags with a true value, and rest of the flags with a false value without changing the tests:

#define ABC20 0
#define ABC10 1
#define ABC00 0