As I understand it, the standard way to implement generic data types in C is with void pointers. However, an alternate approach would be to use macros. Here’s an example implementation of a generic “Option” type using macros:

#define OPTION(T)                                                              
  struct option_##T {                                                          
    bool exists;                                                               
    T data;                                                                    
  };                                                                           
  typedef struct option_##T option_##T;                                        
  bool is_some(option_##T x) { return x.exists; }                              
  bool is_none(option_##T x) { return !x.exists; }

OPTION(int);

int main(int argc, char *argv[]) {
    option_int x = {.exists = true, .data = 3};
}

This has the benefit of avoiding an extra layer of indirection that would be present with a void * implementation, possibly at the cost of binary size. Is there a reason this technique is not used often?