#define hash_hash # ## #
#define mkstr(a) # a
#define in_between(a) mkstr(a)
#define join(c, d) in_between(c hash_hash d)
char p[] = join(x, y); // equivalent to char p[] = "x ## y";
join(x, y) in_between(x hash_hash y) in_between(x ## y) mkstr(x ## y) "x ## y"In other words, expanding hash_hash produces a new token, consisting of two adjacent sharp signs, but this new token is not the ## operator.