Question

I’m trying to create a Breakout clone using C++, and so have several objects (like ball, paddle, powerupicon, block, etc). I understand that it’s bad practice to have them at global scope, so they’re initialized inside main(). The problem comes in with needing to do stuff with those objects from inside other functions (like redraw(), or reset_game()). The only method I can think of to do this would be to pass references of the objects into each function, but I’m not sure if this is the best approach.

Basically, what’s the best way to do stuff with these objects from within a function? Pass them by reference? Make them global but within a namespace? Or something completely different?

Answer 1

A simple (not necessarily flexible or powerful) approach is to define a base game_object class that defines your interface with game objects, and store those. Your objects inherit from it.:

class game_object
{
public:
    virtual ~game_object() {}

    virtual void update() = 0;
    virtual void draw() = 0;

    // get and set position
    virtual vector3 position() = 0;
    virtual void position(const vector3&) = 0;

    // etc...
};

class ball : public game_object
{
public:
    void update() { /* update the ball */ }
    void draw() { /* draw the ball */ }

    // etc
};

// etc

Now you have a common way of using a game object. Next, you can store these in a map (preferably an unordered_map if you have Boost, TR1, or C++0x), and make that map globally available:

// game_object_manager.h
typedef std::map<std::string, game_object*> game_object_manager;

extern game_object_manager manager;

You define this in a translation unit (either main.cpp or a game_object_manager.cpp) somewhere. Lastly, you use it by inserting things by name:

// somewhere, add the ball
manager.insert(std::make_pair("ball", new ball()));

And use it:

game_object* b = manager["ball"];
if (!b) /* there is no ball object */

// use b as ball */

Again, this is a simple solution and isn’t necessarily robust, a best practice, or flexible. But for a first game, it’ll work fine. (To improve it, you want to look into exception safety, smart pointers, and other things (game books). I recommend you do this before you try to make games.)