Generic db impl, in C

generics.c

#include <stdlib.h>                                                             
#include <stdio.h>                                                                 
                                                                                   
// db.h                                                                         
typedef struct DB {                                                             
  void *private;                                                                
  const char *(*get)(struct DB*);                                               
} DB;                                                                           
                                                                                
// test_db_impl.c                                                               
typedef struct {                                                                
  const char *tmpfile;                                                          
} TestDBImpl;                                                                   
                                                                                
void test_db_init(TestDBImpl *t) {                                              
  t->tmpfile = "antempfile.db";                                                 
}                                                                               
                                                                                
const char *test_db_get(DB *db) {                                               
  TestDBImpl *t = (TestDBImpl *)db->private;                                    
  return t->tmpfile;                                                            
}                                                                               
                                                                                
// s3_db_impl.c                                                                 
typedef struct {                                                                
  const char *addr;                                                             
} S3DBImpl;                                                                     
                                                                                
void s3_db_init(S3DBImpl *s) {                                                  
  s->addr = "localhost:3000";                                                   
}                                                                               
                                                                                
const char *s3_db_get(DB *db) {                                                 
  S3DBImpl *s = (S3DBImpl *)db->private;                                        
  return s->addr;                                                               
}                                                                               
                                                                                
// db.c                                                                         
const char *do_something_with_db(DB *db) {                                      
  return db->get(db);                                                           
}                                                                               
                                                                                
// main                                                                         
int main(void) {                                                                
  TestDBImpl test_db_impl;                                                      
  test_db_init(&test_db_impl);                                                  
                                                                                
  S3DBImpl s3_db_impl;                                                          
  s3_db_init(&s3_db_impl);                                                      
                                                                                
  DB test_db;  test_db.get = test_db_get;                                                    
  test_db.private = &test_db_impl;                                              
                                                                                
  DB s3_db;                                                                     
  s3_db.get = s3_db_get;                                                        
  s3_db.private = &s3_db_impl;                                                  
                                                                                
  printf("test_db: %s\n", do_something_with_db(&test_db));                      
  printf("s3_db: %s\n", do_something_with_db(&s3_db));                          
                                                                                
  return EXIT_SUCCESS;                                                       
}