diff --git a/ColoredSquare.c b/ColoredSquare.c
index 52f068b..76903da 100644
--- a/ColoredSquare.c
+++ b/ColoredSquare.c
@@ -3,7 +3,7 @@
 
 #include "ColoredSquare.h"
 
-void ColoredSquare_constructor (void *this,
+void ColoredSquare_constructor (ColoredSquare *this,
                                 double a,
                                 int color)
 {
@@ -11,20 +11,20 @@ void ColoredSquare_constructor (void *this,
           (unsigned long) this,
           a,
           color);
-  Square_constructor (this, a);
+  Square_constructor ((Square *) this, a);
   ((ColoredSquare *) this)->color = color;
 }
 
-void ColoredSquare_copy (void *to, void *from)
+void ColoredSquare_copy (ColoredSquare *to, ColoredSquare *from)
 {
   printf ("ColoredSquare_copy (%lu, %lu) called\n",
           (unsigned long) to,
           (unsigned long) from);
-  Square_copy (to, from);
+  Square_copy ((Square *) to, (Square *) from);
   ((ColoredSquare *) to)->color = ((ColoredSquare *) from)->color;
 }
 
-void* ColoredSquare_clone (void *this)
+ColoredSquare* ColoredSquare_clone (ColoredSquare *this)
 {
   ColoredSquare *csquare = ColoredSquare_new (0.0, 0);
   printf ("ColoredSquare_clone (%lu) called\n",
@@ -37,7 +37,7 @@ void* ColoredSquare_clone (void *this)
   return csquare;
 }
 
-const char* ColoredSquare_type (void *this)
+const char* ColoredSquare_type (ColoredSquare *this)
 {
   static const char *csquare_type_str = "ColoredSquare";
   printf ("ColoredSquare_type (%lu) called\n",
@@ -48,7 +48,7 @@ const char* ColoredSquare_type (void *this)
   return csquare_type_str;
 }
 
-void ColoredSquare_draw (void *this)
+void ColoredSquare_draw (ColoredSquare *this)
 {
   printf ("ColoredSquare_draw (%lu) called\n",
           (unsigned long) this);
@@ -57,7 +57,7 @@ void ColoredSquare_draw (void *this)
           ((ColoredSquare *) this)->color);
 }
 
-void ColoredSquare_set_color (void *this, int color)
+void ColoredSquare_set_color (ColoredSquare *this, int color)
 {
   printf ("ColoredSquare_draw (%lu, %d) called\n",
           (unsigned long) this,
@@ -67,7 +67,7 @@ void ColoredSquare_set_color (void *this, int color)
 }
 
 /* public */
-void* ColoredSquare_new (double a, int color)
+ColoredSquare* ColoredSquare_new (double a, int color)
 {
   static ColoredSquare_interface vtable =
     {
@@ -89,7 +89,7 @@ void* ColoredSquare_new (double a, int color)
           color,
           (unsigned long) square); 
 
-  square->vtable = (void *) &vtable;
+  square->vtable = (ColoredSquare_interface *) &vtable;
 
 /*goto end;
 err:
diff --git a/ColoredSquare.h b/ColoredSquare.h
index 8411bb0..41fdce6 100644
--- a/ColoredSquare.h
+++ b/ColoredSquare.h
@@ -11,22 +11,24 @@ typedef struct ColoredSquare_data
 
 } ColoredSquare_data;
 
+struct ColoredSquare;
+
 typedef struct ColoredSquare_interface
 {
   Square_interface;
 
-  void (*set_color) (void *this, int color);
+  void (*set_color) (struct ColoredSquare *this, int color);
 
 } ColoredSquare_interface;
 
-void ColoredSquare_constructor (void *this,
+void ColoredSquare_constructor (struct ColoredSquare *this,
                                 double a,
                                 int color);
-void ColoredSquare_copy (void *to, void *from);
-void* ColoredSquare_clone (void *this);
-const char* ColoredSquare_type (void *this);
-void ColoredSquare_draw (void *this);
-void ColoredSquare_set_color (void *this, int color);
+void ColoredSquare_copy (struct ColoredSquare *to, struct ColoredSquare *from);
+struct ColoredSquare* ColoredSquare_clone (struct ColoredSquare *this);
+const char* ColoredSquare_type (struct ColoredSquare *this);
+void ColoredSquare_draw (struct ColoredSquare *this);
+void ColoredSquare_set_color (struct ColoredSquare *this, int color);
 
 /* public */
 typedef struct ColoredSquare
@@ -37,6 +39,6 @@ typedef struct ColoredSquare
 
 } ColoredSquare;
 
-void* ColoredSquare_new (double a, int color);
+ColoredSquare* ColoredSquare_new (double a, int color);
 
 #endif // __COLORED_SQUARE_H__
diff --git a/Figure.h b/Figure.h
index 3b3a19a..5c2492c 100644
--- a/Figure.h
+++ b/Figure.h
@@ -5,13 +5,15 @@
 
 #include "Object.h"
 
+struct Figure;
+
 typedef struct Figure_interface
 {
   Object_interface;
 
-  const char* (*type) (void *this);
-  void (*draw) (void *this);
-  double (*area) (void *this);
+  const char* (*type) (struct Figure *this);
+  void (*draw) (struct Figure *this);
+  double (*area) (struct Figure *this);
 
 } Figure_interface;
 
diff --git a/Square.c b/Square.c
index 577d226..abbdd9e 100644
--- a/Square.c
+++ b/Square.c
@@ -2,7 +2,7 @@
 
 #include "Square.h"
 
-void Square_constructor (void *this, double a)
+void Square_constructor (Square *this, double a)
 {
   printf ("Square_constructor (%lu, %f) called\n",
           (unsigned long) this,
@@ -10,13 +10,13 @@ void Square_constructor (void *this, double a)
   ((Square *) this)->a = a;
 }
 
-void Square_destructor (void *this)
+void Square_destructor (Square *this)
 {
   printf ("Square_destructor (%lu) called\n",
           (unsigned long) this);
 }
 
-void Square_copy (void *to, void *from)
+void Square_copy (Square *to, Square *from)
 {
   printf ("Square_copy (%lu, %lu) called\n",
           (unsigned long) to,
@@ -24,7 +24,7 @@ void Square_copy (void *to, void *from)
   ((Square *) to)->a = ((Square *) from)->a;
 }
 
-void* Square_clone (void *this)
+Square* Square_clone (Square *this)
 {
   Square *square = Square_new (0.0);
   printf ("Square_clone (%lu) called\n",
@@ -37,14 +37,14 @@ void* Square_clone (void *this)
   return square;
 }
 
-void Square_destroy (void *this)
+void Square_destroy (Square *this)
 {
   printf ("Square_destroy (%lu) called\n",
           (unsigned long) this);
   free (this);
 }
 
-const char* Square_type (void *this)
+const char* Square_type (Square *this)
 {
   static const char *square_type_str = "Square";
   printf ("Square_type (%lu) called\n",
@@ -56,7 +56,7 @@ const char* Square_type (void *this)
   return square_type_str;
 }
 
-void Square_draw (void *this)
+void Square_draw (Square *this)
 {
   printf ("Square_draw (%lu) called\n",
           (unsigned long) this);
@@ -64,7 +64,7 @@ void Square_draw (void *this)
           ((Square *) this)->a);
 }
 
-double Square_area (void *this)
+double Square_area (Square *this)
 {
   double area = ((Square *)this)->a * ((Square *)this)->a;
   printf ("Square_area (%lu) called\n",
@@ -76,7 +76,7 @@ double Square_area (void *this)
   return area;
 }
 
-void Square_resize (void *this, double a)
+void Square_resize (Square *this, double a)
 {
   printf ("Square_resize (%lu, %f) called\n",
           (unsigned long) this,
@@ -84,7 +84,7 @@ void Square_resize (void *this, double a)
   ((Square *) this)->a = a;
 }
 
-double Square_diag_length (void *this)
+double Square_diag_length (Square *this)
 {
   double diag_length = ((Square *)this)->a * 1.41421356;
 
@@ -98,7 +98,7 @@ double Square_diag_length (void *this)
 }
 
 /* public */
-void* Square_new (double a)
+Square* Square_new (double a)
 {
   static Square_interface vtable =
     {
@@ -118,7 +118,7 @@ void* Square_new (double a)
           a,
           (unsigned long) square); 
 
-  square->vtable = (void *) &vtable;
+  square->vtable = (Square_interface *) &vtable;
 
 /*goto end;
 err:
diff --git a/Square.h b/Square.h
index 36252a6..dcf09cf 100644
--- a/Square.h
+++ b/Square.h
@@ -9,25 +9,27 @@ typedef struct Square_data
 
 } Square_data;
 
+struct Square;
+
 typedef struct Square_interface
 {
   Figure_interface;
 
-  void (*resize) (void *this, double a);
-  double (*diag_length) (void *this);
+  void (*resize) (struct Square *this, double a);
+  double (*diag_length) (struct Square *this);
 
 } Square_interface;
 
-void Square_constructor (void *this, double a);
-void Square_destructor (void *this);
-void Square_copy (void *to, void *from);
-void* Square_clone (void *this);
-void Square_destroy (void *this);
-const char* Square_type (void *this);
-void Square_draw (void *this);
-double Square_area (void *this);
-void Square_resize (void *this, double a);
-double Square_diag_length (void *this);
+void Square_constructor (struct Square *this, double a);
+void Square_destructor (struct Square *this);
+void Square_copy (struct Square *to, struct Square *from);
+struct Square* Square_clone (struct Square *this);
+void Square_destroy (struct Square *this);
+const char* Square_type (struct Square *this);
+void Square_draw (struct Square *this);
+double Square_area (struct Square *this);
+void Square_resize (struct Square *this, double a);
+double Square_diag_length (struct Square *this);
 
 /* public */
 typedef struct Square
@@ -38,6 +40,6 @@ typedef struct Square
 
 } Square;
 
-void* Square_new (double a);
+struct Square* Square_new (double a);
 
 #endif // __SQUARE_H__
diff --git a/main.c b/main.c
index f2a9d8b..cd65bf1 100644
--- a/main.c
+++ b/main.c
@@ -5,18 +5,18 @@
 
 int main (int argc, char *argv[])
 {
-  int i = 0;
+  unsigned long i = 0;
   Figure *fig[3];
 
-  fig[0] = Square_new (1);
-  fig[1] = ColoredSquare_new (2, 2);
+  fig[0] = (Figure *) Square_new (1);
+  fig[1] = (Figure *) ColoredSquare_new (2, 2);
 
-  ((ColoredSquare *) fig[1])->vtable->set_color (fig[1], 5);
-  ((Square *) fig[0])->vtable->resize (fig[0], 2);
-  ((Square *) fig[1])->vtable->resize (fig[0], 3);
+  ((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 (fig[2], 3);
+  ((ColoredSquare *) fig[2])->vtable->set_color ((ColoredSquare *) fig[2], 3);
 
   for (i = 0; i < sizeof (fig) / sizeof (Figure *); i++)
     {
@@ -32,7 +32,7 @@ int main (int argc, char *argv[])
 
       printf ("diag_length(%lu) = %f\n",
               (unsigned long) fig[i],
-              ((Square *) fig[i])->vtable->diag_length (fig[i]));
+              ((Square *) fig[i])->vtable->diag_length ((Square *) fig[i]));
     }
     puts ("---");