main.c

#include "string.h"

void main(int ac, char **args) {
  String hello = string_init("hello");
  hello->append(hello, " my girl");
  printf("got a length of %d now", hello->length(hello));
}

string.c

#include "string.h"

static void length(String sef);
static void append(String self, String to_happend);

String string_init(char *str) {
  String s = malloc(sizeof(*String));
  s->length = &length;
  s->append = &append;
}

static void length(String self) {
  return (strlen(self->str)); 
}

static void append(String self, char *str) {
  free(self->str);
  self->str = strcat(self->str, str);
}

string.h

typedef struct  s_string
{
  char  *str;
  void  (*length)(s_string *self);
  void  (*append)(s_string *self, char *str);
} *String;