Yes! This is very nice API!

Polygon.c

@@ -37,6 +37,7 @@ static Polygon* __Polygon_clone (const Polygon *this)
   Polygon *poly = malloc (sizeof (Polygon));
   memset (poly, 0, sizeof (Polygon));
   Polygon_copy (poly, this);
+  poly->vtable = this->vtable;
   return poly;
 }
 
@@ -54,7 +55,7 @@ void __Polygon_draw (const Polygon *this)
     {
       printf ("{%f;%f}, ", this->points[i].x, this->points[i].y);
     }
-  printf ("{%f;%f}, ", this->points[0].x, this->points[0].y);
+  printf ("{%f;%f}\n", this->points[0].x, this->points[0].y);
 }
 
 double __Polygon_area (const Polygon *this)

Rhomb.c

@@ -31,6 +31,7 @@ static Rhomb* __Rhomb_clone (const Rhomb *this)
   Rhomb *rhomb = malloc (sizeof (Rhomb));
   memset (rhomb, 0, sizeof (Rhomb));
   Rhomb_copy (rhomb, this);
+  rhomb->vtable = this->vtable;
   return rhomb;
 }
 
@@ -67,7 +68,7 @@ static int __Rhomb_is_square (const Rhomb *this)
   tmp2 = this->points[1].y - this->points[3].y;
   diag2 = tmp1 * tmp1 + tmp2 * tmp2;
 
-  return (diag1 == diag2);
+  return (fabs (diag1 - diag2) < 1e-12);
 }
 
 /* public */

main.c

@@ -4,46 +4,82 @@
 
 int main (void)
 {
-  Polygon_destroy (NULL);
-/*  unsigned long i = 0;
+  unsigned long i = 0;
   Figure *fig[3];
+  static struct point points1[] = {
+    {
+      3.1,
+      2.8
+    },
+    {
+      8.1,
+      -3.1
+    },
+    {
+      -1.88,
+      2.11
+    },
+    {
+      3.0,
+      4.1
+    },
+  };
+  static struct point points2[] = {
+    {
+      8.2,
+      4.2
+    },
+    {
+      1.1,
+      2.2
+    },
+    {
+      7.7,
+      5.6
+    },
+    {
+      -12.35,
+      -8.3
+    },
+    {
+      -11.13,
+      -1.1
+    },
+    {
+      2.3,
+      -8.5
+    },
+  };
 
-  fig[0] = (Figure *) Square_new (1);
-  fig[1] = (Figure *) ColoredSquare_new (2, 2);
+  fig[0] = (Figure *) Polygon_new (points2, sizeof (points2) / sizeof (points2[0]));
+  fig[1] = (Figure *) Rhomb_new (points1);
 
-  ((ColoredSquare *) fig[1])->vtable->set_color ((ColoredSquare *) fig[1], 5);
-  ((Square *) fig[0])->vtable->resize ((Square *) fig[0], 2);
-  ((Square *) fig[1])->vtable->resize ((Square *) fig[0], 3);
-
-  fig[2] = fig[1]->vtable->clone (fig[1]);
-  ((ColoredSquare *) fig[2])->vtable->set_color ((ColoredSquare *) fig[2], 3);
+  printf ("Is rhomb square? %d\n", Rhomb_is_square ((Rhomb *) fig[1]));
+  printf ("Max diag = %f\n", Polygon_max_diag ((Polygon *) fig[0]));
+  fig[2] = Figure_clone (fig[1]);
+  printf ("Area = %f\n", Figure_area (fig[2]));
 
   for (i = 0; i < sizeof (fig) / sizeof (Figure *); i++)
     {
       puts ("---");
 
-      printf ("type = %s\n", fig[i]->vtable->type (fig[i]));
+      printf ("type = %s\n", Figure_type (fig[i]));
 
-      fig[i]->vtable->draw (fig[i]);
+      Figure_draw (fig[i]);
 
       printf ("area(%lu) = %f\n",
               (unsigned long) fig[i],
-              fig[i]->vtable->area (fig[i]));
-
-      printf ("diag_length(%lu) = %f\n",
-              (unsigned long) fig[i],
-              ((Square *) fig[i])->vtable->diag_length ((Square *) fig[i]));
+              Figure_area (fig[i]));
     }
     puts ("---");
 
 /*goto end;
 err:
 end:*/
-/*  for (i = 0; i < sizeof (fig) / sizeof (Figure *); i++)
+  for (i = 0; i < sizeof (fig) / sizeof (Figure *); i++)
     {
-      fig[i]->vtable->destroy (fig[i]);
+      Figure_destroy (fig[i]);
     }
-*/
 
   return 0;
 }