Before we discussed the tower and enemy behaviors, we parsed an XML file containing the various hordes or waves of enemies that the player would have to face in a given level. Now it's time to code in the logic of actually spawning a wave and its constituent enemies. A wave may be spawned after any previous wave has completed or when the level has just begun. Let's take a look at the spawning of a wave in the StartNextWave function of GameWorld.cpp:

void GameWorld::StartNextWave(float dt)
{
  // increment the current wave index
  ++ curr_wave_index_;
  // are there any waves left?
  if(curr_wave_index_ >= num_waves_)
  {
    // level has completed
    GameOver(true);
  }
  else
  {
    // start the next wave in a few seconds
    curr_wave_ = waves_[curr_wave_index_];
    schedule(schedule_selector(GameWorld::SpawnEnemy), 
      curr_wave_->spawn_delay_);
    UpdateHUD();

    GameGlobals::ScaleLabel(waves_label_);
  }
}

The reason that the StartNextWave function...