foo.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct person person;

struct person
{
  /* The data fields */
  char *first_name;
  char *last_name;
  int age;
  
  /* The methods. */
  void (*set_first_name)(person *, char *);
  char* (*get_first_name)(person *);
  void (*set_last_name)(person *, char *);
  char* (*get_last_name)(person *);
  void (*set_age)(person *, int);
  int (*get_age)(person *);
};

/* The variable 'o' in these functions is in effect 'this' in C++ or also
   'self' in python. Single DIspatch only, in this context.
*/
void set_first_name(person *o, char *val)
{
  if (o->first_name != NULL) {
    free(o->first_name);
  }
  o->first_name = strdup(val); 
}

char* get_first_name(person *o) { return o->first_name; }

void set_last_name(person *o, char *val)
{
  if (o->last_name != NULL) {
    free(o->last_name);
  }
  o->last_name = strdup(val);
}

char* get_last_name(person *o) { return o->last_name; }
void set_age(person *o, int val) { o->age = val; }
int get_age(person *o) { return o->age; }

/* person constructor */
person* make_person(char *first_name, char *last_name, int age)
{
  person *o = (person*)malloc(sizeof(person) * 1);
  if (o == NULL) {
    printf("Out of memory!\n");
    exit(EXIT_FAILURE);
  }
  
  /* fundamental field initialization to valid values. */
  o->first_name = NULL;
  o->last_name = NULL;
  o->age = 0;
  
  /* connect the methods into the instance*/
  o->set_first_name = set_first_name;
  o->get_first_name = get_first_name;
  o->set_last_name = set_last_name;
  o->get_last_name = get_last_name;
  o->set_age = set_age;
  o->get_age = get_age;

  /* Now initalize the instance with the method functions and constructor values. */
  o->set_first_name(o, first_name);
  o->set_last_name(o, last_name);
  o->set_age(o, age);

  return o;
}

void free_person(person *o)
{
  if (o != NULL) {
    free(o->first_name);
    free(o->last_name);
  }
  
  free(o);
}

int main(void)
{
  person *p0, *p1;

  p0 = make_person("Fred", "Baily", 44);
  p1 = make_person("Jane", "Smith", 45);

  printf("The first person is:\n");
  printf(" First Name: %s\n", p0->get_first_name(p0));
  printf(" Last Name: %s\n", p0->get_last_name(p0));
  printf(" Age: %d\n", p0->get_age(p0));

  printf("The second person is:\n");
  printf(" First Name: %s\n", p1->get_first_name(p1));
  printf(" Last Name: %s\n", p1->get_last_name(p1));
  printf(" Age: %d\n", p1->get_age(p1));

  free_person(p0);
  free_person(p1);

  return EXIT_SUCCESS;
}