interface_model.c

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

typedef void (*cleanup_function) (void*);

// the interface
struct IPoint {
  float *x, *y;
  cleanup_function cleanup;
};

void ipoint_print (struct IPoint ip) {
  printf("x: %f, y: %f\n", *ip.x, *ip.y);
}
void ipoint_cleanup (struct IPoint *ip){
  if (ip->cleanup) 
    ((void (*)(struct IPoint *))(ip->cleanup))(ip);
}
void cleanup_ipoint (struct IPoint *ip) { free(ip->x); free(ip->y); }


typedef struct { float x, y; } Point;
// use point as IPoint
struct IPoint point_ipoint (Point *p) {
  struct IPoint result;
  result.x = &p->x;
  result.y = &p->y;
  result.cleanup = 0;
  return result;
}


typedef struct{ float left,top,w,h; } Rect;
// use rect as IPoint
struct IPoint rect_ipoint (Rect *r) {
  float *x = (float*)malloc(sizeof(float));
  *x = r->left + (r->w / 2);
  float *y = (float*)malloc(sizeof(float));
  *y = r->top + (r->h / 2);
  
  struct IPoint result;
  result.x = x;
  result.y = y;
  result.cleanup = (cleanup_function)(&cleanup_ipoint);
  
  return result;
}

int main(void) {
  Point p = {10,20};
  struct IPoint ip = point_ipoint(&p);
  ipoint_print(ip);
  ipoint_cleanup(&ip);
  
  Rect r = {0,0,50,100};
  ip = rect_ipoint(&r);
  ipoint_print(ip);
  ipoint_cleanup(&ip);
  
  return 0;
}