Ron Beyer is right in what he says in solution 1. Any static data members of a class must somewhere be defined. You just need to add a line
vector<city> TourManager::destinationCities;
somewhere in a cpp file. You are correct in assuming that a vector knows how to initialize itself. That applies if you define a vector as an instance member of a class. In your case, however, you are declaring a static members, and all static data members must be defined somewhere. That is the reason it worked in your previous experience and why it didn't in this particular case.