validation/preprocessor3.c

   1 /*
   2  * We get this one wrong too.
   3  *
   4  * It should result in a sequence
   5  *
   6  *      B ( )
   7  *      A ( )
   8  *      B ( )
   9  *      A ( )
  10  *
  11  * because each iteration of the scanning of "SCAN()"
  12  * should re-evaluate the recursive B->A->B expansion.
  13  * But we never re-evaluate something that we noticed
  14  * was recursive. So we will cause it to evaluate to
  15  *
  16  *      B ( )
  17  *      A ( )
  18  *      A ( )
  19  *      A ( )
  20  *
  21  * Which is really quite wrong.
  22  *
  23  * Did I already mention that the C preprocessor language
  24  * is a perverse thing?
  25  */
  26
  27 #define LP (
  28
  29 #define A() B LP )
  30 #define B() A LP )
  31
  32 #define SCAN(x) x
  33
  34 A()                     // B ( )
  35 SCAN( A() )             // A ( )
  36 SCAN(SCAN( A() ))       // B ( )
  37 SCAN(SCAN(SCAN( A() ))) // A ( )