Error(s):

source_file.cpp:8:41: error: pasting "FIRST" and "(" does not give a valid preprocessing token
 #define FIRST_OF_PAIR(Pair)     _GLUE2( FIRST, (Pair) )
                                         ^
source_file.cpp:6:22: note: in definition of macro ‘_GLUE2’
 #define _GLUE2(a, b) a ## b
                      ^
source_file.cpp:13:13: note: in expansion of macro ‘FIRST_OF_PAIR’
     int a = FIRST_OF_PAIR(PAIR);
             ^
source_file.cpp:9:41: error: pasting "SECOND" and "(" does not give a valid preprocessing token
 #define SECOND_OF_PAIR(Pair)    _GLUE2( SECOND, (Pair) )
                                         ^
source_file.cpp:6:22: note: in definition of macro ‘_GLUE2’
 #define _GLUE2(a, b) a ## b
                      ^
source_file.cpp:14:13: note: in expansion of macro ‘SECOND_OF_PAIR’
     int b = SECOND_OF_PAIR(PAIR);
             ^
source_file.cpp: In function ‘int main()’:
source_file.cpp:14:9: warning: unused variable ‘b’ [-Wunused-variable]
     int b = SECOND_OF_PAIR(PAIR);
         ^