I have a basic 2D tower defense game in C++.

Each map is a separate class which inherits from GameState. The map delegates the logic and drawing code to each object in the game and sets data such as the map path. In pseudo-code the logic section might look something like this:

update():
  for each creep in creeps:
    creep.update()
  for each tower in towers:
    tower.update()
  for each missile in missiles:
    missile.update()

The objects (creeps, towers and missiles) are stored in vector-of-pointers. The towers must have access to the vector-of-creeps and the vector-of-missiles to create new missiles and identify targets.

The question is: where do I declare the vectors? Should they be members of the Map class, and passed as arguments to the tower.update() function? Or declared globally? Or are there other solutions I'm missing entirely?