diff --git a/ColoredSquare.c b/ColoredSquare.c
index e30998d..3b549c6 100644
--- a/ColoredSquare.c
+++ b/ColoredSquare.c
@@ -15,7 +15,7 @@ void ColoredSquare_constructor (ColoredSquare *this,
   this->color = color;
 }
 
-void ColoredSquare_copy (ColoredSquare *to, ColoredSquare *from)
+void ColoredSquare_copy (ColoredSquare *to, const ColoredSquare *from)
 {
   printf ("ColoredSquare_copy (%lu, %lu) called\n",
           (unsigned long) to,
@@ -24,7 +24,7 @@ void ColoredSquare_copy (ColoredSquare *to, ColoredSquare *from)
   to->color = from->color;
 }
 
-ColoredSquare* ColoredSquare_clone (ColoredSquare *this)
+ColoredSquare* ColoredSquare_clone (const ColoredSquare *this)
 {
   ColoredSquare *csquare = ColoredSquare_new (0.0, 0);
   printf ("ColoredSquare_clone (%lu) called\n",
@@ -37,7 +37,7 @@ ColoredSquare* ColoredSquare_clone (ColoredSquare *this)
   return csquare;
 }
 
-const char* ColoredSquare_type (ColoredSquare *this)
+const char* ColoredSquare_type (const ColoredSquare *this)
 {
   static const char *csquare_type_str = "ColoredSquare";
   printf ("ColoredSquare_type (%lu) called\n",
@@ -48,7 +48,7 @@ const char* ColoredSquare_type (ColoredSquare *this)
   return csquare_type_str;
 }
 
-void ColoredSquare_draw (ColoredSquare *this)
+void ColoredSquare_draw (const ColoredSquare *this)
 {
   printf ("ColoredSquare_draw (%lu) called\n",
           (unsigned long) this);
@@ -74,12 +74,12 @@ ColoredSquare* ColoredSquare_new (double a, int color)
       {
         {
           {
-            (void* (*) (void *)) ColoredSquare_clone,
+            (void* (*) (const void *)) ColoredSquare_clone,
             (void (*) (void *)) Square_destroy
           },
-          (const char* (*) (Figure *)) ColoredSquare_type,
-          (void (*) (Figure *)) ColoredSquare_draw,
-          (double (*) (Figure *)) Square_area
+          (const char* (*) (const Figure *)) ColoredSquare_type,
+          (void (*) (const Figure *)) ColoredSquare_draw,
+          (double (*) (const Figure *)) Square_area
         },
         Square_resize,
         Square_diag_length
diff --git a/ColoredSquare.h b/ColoredSquare.h
index 41fdce6..c57e012 100644
--- a/ColoredSquare.h
+++ b/ColoredSquare.h
@@ -24,10 +24,10 @@ typedef struct ColoredSquare_interface
 void ColoredSquare_constructor (struct ColoredSquare *this,
                                 double a,
                                 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_copy (struct ColoredSquare *to, const struct ColoredSquare *from);
+struct ColoredSquare* ColoredSquare_clone (const struct ColoredSquare *this);
+const char* ColoredSquare_type (const struct ColoredSquare *this);
+void ColoredSquare_draw (const struct ColoredSquare *this);
 void ColoredSquare_set_color (struct ColoredSquare *this, int color);
 
 /* public */
diff --git a/Figure.h b/Figure.h
index f67462d..7a0eecc 100644
--- a/Figure.h
+++ b/Figure.h
@@ -9,9 +9,9 @@ typedef struct Figure_interface
 {
   Object_interface;
 
-  const char* (*type) (struct Figure *this);
-  void (*draw) (struct Figure *this);
-  double (*area) (struct Figure *this);
+  const char* (*type) (const struct Figure *this);
+  void (*draw) (const struct Figure *this);
+  double (*area) (const struct Figure *this);
 
 } Figure_interface;
 
diff --git a/Object.h b/Object.h
index e8f96a0..66a1d08 100644
--- a/Object.h
+++ b/Object.h
@@ -3,7 +3,7 @@
 
 typedef struct Object_interface
 {
-  void* (*clone) (void *this);
+  void* (*clone) (const void *this);
   void (*destroy) (void *this);
 
 } Object_interface;
diff --git a/Square.c b/Square.c
index 1fc5e8a..83f298b 100644
--- a/Square.c
+++ b/Square.c
@@ -17,7 +17,7 @@ void Square_destructor (Square *this)
           (unsigned long) this);
 }
 
-void Square_copy (Square *to, Square *from)
+void Square_copy (Square *to, const Square *from)
 {
   printf ("Square_copy (%lu, %lu) called\n",
           (unsigned long) to,
@@ -25,7 +25,7 @@ void Square_copy (Square *to, Square *from)
   to->a = from->a;
 }
 
-Square* Square_clone (Square *this)
+Square* Square_clone (const Square *this)
 {
   Square *square = Square_new (0.0);
   printf ("Square_clone (%lu) called\n",
@@ -45,7 +45,7 @@ void Square_destroy (Square *this)
   free (this);
 }
 
-const char* Square_type (Square *this)
+const char* Square_type (const Square *this)
 {
   static const char *square_type_str = "Square";
   printf ("Square_type (%lu) called\n",
@@ -57,7 +57,7 @@ const char* Square_type (Square *this)
   return square_type_str;
 }
 
-void Square_draw (Square *this)
+void Square_draw (const Square *this)
 {
   printf ("Square_draw (%lu) called\n",
           (unsigned long) this);
@@ -65,7 +65,7 @@ void Square_draw (Square *this)
           this->a);
 }
 
-double Square_area (Square *this)
+double Square_area (const Square *this)
 {
   double area = this->a * this->a;
   printf ("Square_area (%lu) called\n",
@@ -85,7 +85,7 @@ void Square_resize (Square *this, double a)
   this->a = a;
 }
 
-double Square_diag_length (Square *this)
+double Square_diag_length (const Square *this)
 {
   double diag_length = ((Square *)this)->a * 1.41421356;
 
@@ -105,12 +105,12 @@ Square* Square_new (double a)
     {
       {
         {
-          (void* (*) (void *)) Square_clone,
+          (void* (*) (const void *)) Square_clone,
           (void (*) (void *)) Square_destroy
         },
-        (const char* (*) (Figure *)) Square_type,
-        (void (*) (Figure *)) Square_draw,
-        (double (*) (Figure *)) Square_area
+        (const char* (*) (const Figure *)) Square_type,
+        (void (*) (const Figure *)) Square_draw,
+        (double (*) (const Figure *)) Square_area
       },
       Square_resize,
       Square_diag_length
diff --git a/Square.h b/Square.h
index 5d5042f..253f681 100644
--- a/Square.h
+++ b/Square.h
@@ -16,20 +16,20 @@ typedef struct Square_interface
   Figure_interface;
 
   void (*resize) (struct Square *this, double a);
-  double (*diag_length) (struct Square *this);
+  double (*diag_length) (const struct Square *this);
 
 } Square_interface;
 
 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_copy (struct Square *to, const struct Square *from);
+struct Square* Square_clone (const 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);
+const char* Square_type (const struct Square *this);
+void Square_draw (const struct Square *this);
+double Square_area (const struct Square *this);
 void Square_resize (struct Square *this, double a);
-double Square_diag_length (struct Square *this);
+double Square_diag_length (const struct Square *this);
 
 /* public */
 typedef struct Square