Getting Physical – Box2D - Cocos2d-x by Example Beginner's Guide - page 307

Time for action - adding the main loop
It's inside our main loop that we update our b2World object.
1. Start by updating the simulation as follows:
void GameLayer::update(float dt) {
if (!_running) return;
if (_gameState == kGameOver) return;
_world->Step(dt, 10, 10);
2. Next, we need to determine if the game has finished by checking on the number of
balls currently in play. We use the following for that:
//track invisible objects
for (auto ball : _balls) {
if (!ball->isVisible() && ball->getInPlay()) {
ball->setInPlay(false);
ball->hide();
//count down balls
_ballsInPlay--;
SimpleAudioEngine::getInstance()->playEffect("drop.wav");
if (_ballsInPlay == 0) {
_gameState = kGameOver;
_gameOver->setVisible(true);
}
} else {
ball->update(dt);
}
}
3. Next, we continue to update the sprites as follows:
if (!_cue->isVisible()) {
_cue->hide();
} else {
_cue->update(dt);
}