Clean up code and add more comments.

src/gameobj.d

@@ -18,25 +18,44 @@
 module gameobj;
 import derelict.sdl2.sdl;
 
-interface GameObj {
-	void prepare();
-	void destroy();
-	void exec();
-	bool wants_to_quit() const;
-}
-
-class GameObjBase : GameObj {
+//We want to provide an interface for all game scenes. A game scene should
+//implement some basic methods, such as prepare() and exec(), so the main loop
+//can call them.
+//We also want our interface to store a renderer. This forces us to declare
+//an abstract class instead of an interface, becaues interfaces can't have
+//constructors nor properties.
+abstract class GameObj {
 public:
 	this (SDL_Renderer* parRenderer) {
 		m_renderer = parRenderer;
 		assert(m_renderer);
 	}
 
+	//Derived classes must implement those;
+	//failure to do so results in a compilation error.
+	//You need to mark them as "abstract" to let the compiler know there is
+	//no implementation for them. If you don't mark them as abstract and you
+	//forget to implement them in the derived game object, you will get a
+	//linker error instead, and potentially the wrong behaviour as you call them
+	//from a pointer to the base class.
+	abstract void prepare();
+	abstract void dispose();
+	abstract void exec();
+	abstract bool wants_to_quit() const;
+
 protected:
-	SDL_Renderer* renderer() {
+	//Allow game objects inheriting from GameObj to retrieve the renderer they
+	//stored through GameObj's constructor. final means it can't be overridden
+	//(so no v-table entry required for this method); @property means you must
+	//not use () when you call renderer on an object, eg: r=my_game_obj.renderer
+	@property final SDL_Renderer* renderer() {
 		return m_renderer;
 	}
 
+//Remember: private members can still be accessed from within this module.
+//Or, from a different point of view if you come from a c++ background, within
+//the gameobj module, everything is friend to everything else.
+//One more good reason for putting each class into a different module.
 private:
 	SDL_Renderer* m_renderer;
 }

src/gameobjjumping.d

@@ -21,8 +21,11 @@ import derelict.sdl2.sdl;
 import texture;
 import std.stdio;
 
-class GameObjJumping : GameObjBase {
-	//Don't disable this(), it's disable automatically when you define a
+//This is one possible game scene. It will draw a texture on the screen
+//and it will report user wants to quit if you press the close button in the
+//window decoration.
+class GameObjJumping : GameObj {
+	//Don't disable this(), it's disabled automatically when you define a
 	//custorm constructor.
 	//@disable this();
 
@@ -32,28 +35,40 @@ class GameObjJumping : GameObjBase {
 		m_wants_to_quit = false;
 	}
 
+	//Remember: class destructors are not guaranteed to be called
+	//deterministically, so all resource-freeing code should rather go in the
+	//dispose() method.
 	~this() {
 		writeln("Destroying GameObj");
+		//m_texture's type is a struct, so destruction is implicit at this point
 	}
 
-	void prepare() {
+	override void prepare() {
 		string texture_name = "bcruiser_normal.bpg";
 		writeln("Loading texture %s", texture_name);
 		m_texture = Texture(texture_name, renderer());
 	}
 
-	void destroy() {
+	override void dispose() {
+		writeln("GameObjJumping dispose()");
+		//m_texture.dispose() is what you would normally expect to find here,
+		//but to demonstrate the use of clear() from the caller code, this
+		//method is intentionally empty.
+		//You can verify the program's output as you quit by uncommenting the
+		//following line:
+		//m_texture.dispose();
 	}
 
-	//This is the function that is called at every frame
-	void exec() {
+	//This is the function that is called at every frame. It represents the
+	//main loop's body.
+	override void exec() {
 		SDL_RenderClear(renderer());
 		m_texture.draw();
 		SDL_RenderPresent(renderer());
 		m_wants_to_quit = do_events();
 	}
 
-	bool wants_to_quit() const {
+	override bool wants_to_quit() const {
 		return m_wants_to_quit;
 	}
 

src/main.d

@@ -21,13 +21,14 @@ import std.string;
 import std.conv;
 import derelict.sdl2.sdl;
 import gameobjjumping;
+import std.typecons : scoped; //Only import scoped from std.typecons module
 
-void run_main_loop (ref GameObj parGame) {
+void run_main_loop (GameObj parGame) {
 	parGame.prepare();
 	do {
 		parGame.exec();
 	} while (!parGame.wants_to_quit());
-	parGame.destroy();
+	parGame.dispose();
 }
 
 int main() {
@@ -53,9 +54,31 @@ int main() {
 	scope(exit) { writeln("Destroying renderer..."); SDL_DestroyRenderer(renderer); }
 
 	//This is our game object
-	GameObj game = new GameObjJumping(renderer);
-	scope(exit) clear(game);
+	//Because GameObjJumping implementation purposedly doesn't destroy resources
+	//in its dispose method (for this example's sake), you must make sure its
+	//destructor gets called. You have several choices:
+	//
+	//1 - call clear() (deprecated and removed in recent D compilers)
+	//  GameObj game = new GameObjJumping(renderer);
+	//  scope(exit) clear(game);
+	//
+	//2 - call destroy()
+	//note that while game.destroy() is semantically equivalent to the
+	//destroy(game) syntax, you should never write it that way in case game
+	//provides its own destroy() method. In that case the method will be
+	//preferred, and you will end up doing something potentially different than
+	//what you intended to. For the same reason, don't provide a destroy()
+	//method in your classes.
+	//  GameObj game = new GameObjJumping(renderer);
+	//  scope(exit) destroy(game);
+	//
+	//3 - use scope (deprecated)
+	//  scope GameObj game = new GameObjJumping(renderer);
+	//
+	//4 - use scoped (no need for new)
+	//  auto game = scoped!GameObjJumping(renderer);
 
+	auto game = scoped!GameObjJumping(renderer);
 	run_main_loop(game);
 	return 0;
 }

src/texture.d

@@ -44,10 +44,10 @@ public:
 
 	~this() {
 		writefln("Destroying texture %s (%s)", m_filename, m_texture);
-		destroy();
+		dispose();
 	}
 
-	void destroy() {
+	void dispose() {
 		if (m_texture) {
 			SDL_DestroyTexture(m_texture);
 			m_texture = null;