C++ Object Oriented Design

Question

class Allegro {}; class Input { private: Allegro allegro1 }; class Graphics { private: Allegro allegro2 }; class Game { private: Input keyBoardinput; Graphics Gamegraphics; };

In the input I need access to Allegro functions related to the user keyboard input. In the Graphics class,I want only functions that will enable me to draw things on the screen, like shapes etc. However the Game class also needs access to Input and Graphics functions while the game is playing.Any ideas on how I can improve the design. I know I can remove the Graphics and Input classes and have all the Allegro functions inside Game class, because what I want to do is to separate implementation and logic and to also avoid long classes that do too many things.

There is no question here. There is nothing to improve, what you have so far is so little that there's no way to tell what might be wrong with it. What you have posted is clearly OOP, but it's such a small example that it's impossible to point out anything good or bad about it. — user229044
– user229044 ♦, Commented Sep 7, 2011 at 21:31
I need to make a design decision, the question is, is there any better way of achieving a similar objective, an example having only the game and Allegro class, and inserting an interface between them. I this way better than having composition relationships illustrated in the code? Is it a good idea/design to have two objects composed on one object? — Obakeng
– Obakeng, Commented Sep 7, 2011 at 21:43
No, a single massive Allegro class wouldn't be better, as you stated in your question, for the reasons you stated in your question. Beyond what you already say you know, there is no way to tell you anything useful based on the stub of code you posted. — user229044
– user229044 ♦, Commented Sep 7, 2011 at 21:53

Nicol Bolas · Accepted Answer · 2011-09-07 21:31:05Z

Obviously, Graphics and Input should be talking to the same Allegro class. So you should do that: create the Allegro class instance, then use that instance in the constructors for Graphics and Input.

Sarfaraz Nawaz · Accepted Answer · 2011-09-07 21:51:09Z

Your Game class should create the instance of Allegro and the same instance should be passed to Input and Graphics, and the latter two will simply hold the reference of the instance.

class Game { Game() : allegro(/*..*/), keyBoardinput(allegro), Gamegraphics(allegro) {} private: Allegro allegro; //actual instance Input keyBoardinput; Graphics Gamegraphics; };

Note that the order of members in Game is important. allegro should be declared before keyBoardinput and Gamegraphics. This makes sure that allegro is created before the rest two, and then you pass allegro (the fully created instance) to the constructors of Input and Graphics.

And then make sure that Input and Graphics hold the reference of allegro which is passed from Game. Don't make a copy of allegro:

class Input { public: Input(Allegro & allegro) : allegro1(allegro) {} private: Allegro & allegro; //reference - not copy! }; class Graphics { public: Graphics(Allegro & allegro) : allegro1(allegro) {} private: Allegro & allegro; //reference - not copy };

Note that all of these classes make use of member-initialization-list which is also very important here.

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