Make a few pure functions in GameObject non-pure.

There's no reason to force derived classes to implement draw() and
update(). We may as well just provide "do nothing" default
implementations for objects that don't care about the passage of time
or are not drawing anything.

Also fix up the types of draw() arguments - references are nicer than
pointers here since they can't be null.

playstate.cc

@@ -182,15 +182,17 @@ void Board::draw(SDL_Surface* screen)
     if (!*it)
       continue;
 
-    (*it)->draw(&surf, &srect);
+    (*it)->draw(surf, srect);
     centerDraw(*it, srect, drect);
-    SDL_BlitSurface(surf, &srect, screen, &drect);
+    if (surf)
+      SDL_BlitSurface(surf, &srect, screen, &drect);
   }
 
   // Draw the player on top
-  m_player->draw(&surf, &srect);
+  m_player->draw(surf, srect);
   centerDraw(m_player, srect, drect);
-  SDL_BlitSurface(surf, &srect, screen, &drect);
+  if (surf)
+    SDL_BlitSurface(surf, &srect, screen, &drect);
 }
 
 std::vector<std::pair<Uint16, Uint16> > Board::freeTiles() const
@@ -325,10 +327,10 @@ void Block::update(Uint32 delta_time)
   }
 }
 
-void Block::draw(SDL_Surface** surface, SDL_Rect* rect)
+void Block::draw(SDL_Surface*& surface, SDL_Rect& rect)
 {
-  *surface = m_current_frame;
-  *rect = m_current_frame_rect;
+  surface = m_current_frame;
+  rect = m_current_frame_rect;
 }
 
 void Block::collision(GameObject* other)
@@ -368,11 +370,11 @@ void Wall::update(Uint32)
 {
 }
 
-void Wall::draw(SDL_Surface** surface, SDL_Rect* rect)
+void Wall::draw(SDL_Surface*& surface, SDL_Rect& rect)
 {
-  *surface = m_current_frame;
+  surface = m_current_frame;
   SDL_Rect r = m_current_frame_rect;
-  *rect = r;
+  rect = r;
 }
 
 void Wall::collision(GameObject* other)
@@ -493,7 +495,7 @@ void Player::update(Uint32 delta_time)
   m_time_since_move = 0;
 }
 
-void Player::draw(SDL_Surface** surface, SDL_Rect* rect)
+void Player::draw(SDL_Surface*& surface, SDL_Rect& rect)
 {
   // clear our frame surface
   Uint32 col = SDL_MapRGBA(m_frame->format, 0, 0, 0, 0);
@@ -556,8 +558,8 @@ void Player::draw(SDL_Surface** surface, SDL_Rect* rect)
   r.y = 0;
   r.w = m_frame->w;
   r.h = m_frame->h;
-  *rect = r;
-  *surface = m_frame;
+  rect = r;
+  surface = m_frame;
 }
 
 void Player::setEffects(const Effect& e)

playstate.hh

@@ -67,14 +67,18 @@ class GameObject {
 public:
   virtual ~GameObject() { }
 
-  virtual void update(Uint32 delta_time) = 0;
+  virtual void update(Uint32 delta_time) { }
   // This draw method is called by the Board class when drawing. This
   // method should not do any actual drawing itself, just return a
   // pointer to the surface to be used a source and a rectangle
   // describing the area of that surface the GameObject wishes to have
   // drawn. The Board class then takes care of centering this on the
   // tile the object occupies and do the actual drawing.
-  virtual void draw(SDL_Surface** surface, SDL_Rect* rect) = 0;
+  virtual void draw(SDL_Surface*& surface, SDL_Rect& rect)
+  {
+    surface = 0;
+    rect.x = rect.y = rect.w = rect.h = 0;
+  }
 
   virtual Uint16 x() const { return m_x; }
   virtual Uint16 y() const { return m_y; }
@@ -110,7 +114,7 @@ public:
   BLOCK_COLOR color() { return m_col; }
 
   virtual void update(Uint32 delta_time);
-  virtual void draw(SDL_Surface** surface, SDL_Rect* rect);
+  virtual void draw(SDL_Surface*& surface, SDL_Rect& rect);
   virtual void collision(GameObject* other);
 private:
   Block(const Block&);
@@ -129,7 +133,7 @@ public:
   virtual ~Wall();
 
   virtual void update(Uint32 delta_time);
-  virtual void draw(SDL_Surface** surface, SDL_Rect* rect);
+  virtual void draw(SDL_Surface*& surface, SDL_Rect& rect);
   virtual void collision(GameObject*);
 
   virtual bool isBlocked() { return true; }
@@ -154,7 +158,7 @@ public:
   ~Player();
 
   void update(Uint32 delta_time);
-  void draw(SDL_Surface** surface, SDL_Rect* rect);
+  void draw(SDL_Surface*& surface, SDL_Rect& rect);
 
   void goUp() { m_direction = UP; }
   void goDown() { m_direction = DOWN; }