// Pong
void Game::onKeyPress( Key pressedKey ) {
  switch( key ) {
    case KP_8: // Keypad 8
      if ( !singlePlayer ) {
        rightPaddle.moveUp();
      }
      break;
    // ...
  }
}

// New game
void Game::onKeyPress( Key pressedKey ) {
  switch( key ) {
    case KP_8:
      switch( gameMode ) {
        case GM_SINGLEPLAYERCLASSIC:
          // action 1
          break;
        case GM_SINGLEPLAYERTIMEATTACK:
          // action 2
          break;
        case GM_TWOPLAYERCLASSIC:
          // action 3
          break;

        // ... MAINTAINANCE HELL
      }
      break;
    // ...
  }
}

class GameMode {protected:    Engine *eHandle;public:    virtual void setEngineHandle(Engine *eHandle) { this->eHandle = eHandle; } //give the "module" a pointer to the engine so that it can affect stuff.    virtual void startGame(); //initialize whatever    virtual void update(float dt); //update gamestate (run each frame or timestep)    virtual void handleEvent(GameEvent e); //handles game events}then create child classes for each gameMode, you could add more functions if you feel like its necessary. such asclass DeathMatchMode : public GameMode {private://whatever variables you need for DMpublic:    void startGame() {        //init the variables etc    }    void update(float dt) {        //do whatever needs to be done here    }    void handleEvent(GameEvent e) {        if (e.getType() == e.SOME_EVENT) {            //do some stuff internally            //run eHandle->someCoolFunction            //etc        }    }}using scripts or dlls instead works in pretty much the same way but has the extra advantage of allowing your users to mod the game. (dlls give the modders more power but will also allow a modder to create a mod containing nasty code)