Use #define preprocessor directive in a weird way

Question

Today I just finish reading and experimenting on C about how to use #define to create a manifest constant, after that something came into my mind, and below is the code.

#include <stdio.h>
#define dummy main
#define yam {
#define apple }

int dummy(void)        //constant substitution main with dummy
yam                          // constant substitution { with yam
  printf("It works!!\n");
  return 0;
apple                           //constant substitution } with apple

As expected, it works like charm, I just wonder why something like that didn't cause any error, maybe I could understand why the main() could be substituted because main is an identifier (name given to a function, variable and constant), but why the {} can be substituted with a symbolic name too? The second thing is, what data type C use to store this symbolic constant which is not a character enclosed in single quote "" nor an integer or floating-point number.

Answer 1

The #define statements are evaluated by a preprocessor before the program is actually compiled, so the compiler never sees yam. The preprocessor performs a direct text substitution.

That is to say, when the compiler sees your code, it looks like this:

int main(void)        //constant substitution main with dummy
{                          // constant substitution { with yam
  printf("It works!!\n");
  return 0;
}                           //constant substitution } with apple