C++ vector issue - 'LNK2001: unresolved external symbol private: static...'

Question

Before anyone calls me out for not looking at pre-existing questions, I have looked and realise that it is to do with declaration, but I still can't get it to work (might be something to do with me using vectors).

Manager.h:

#include "Flight.h"
#ifndef manager_h
#define manager_h

class Manager {
    static vector<Airport> airports;
    static vector<Flight> flights;
public:
    static void loadAirports();
    static void loadFlights();
    static Airport getAirport(string code);
    static vector<string> split(const string &s, vector<string> &elems);
};

#endif

Manager.cpp:

#include "Manager.h"

void Manager::loadAirports ()
{
    ifstream airportfile("airports.txt");
    string line;
    while (getline(airportfile, line))
    {
        vector<string> values;
        split(line, values);
        Airport airport (values[0], values[1], atoi(values[2].c_str()));
        airports.push_back(airport);
    }
}

void Manager::loadFlights ()
{
    ifstream flightfile("flights.txt");
    string line;
    while (getline(flightfile, line))
    {
        vector<string> values;
        split(line, values);
        Flight flight (getAirport(values[0]), getAirport(values[1]), atoi(values[2].c_str()), atoi(values[3].c_str()));
        flights.push_back(flight);
    }
    cout << flights.size() << endl;
}

Airport Manager::getAirport (string code)
{
    for (int i = 1; i < (int)airports.size(); i++)
    {
        if (airports[i].code == code)
            return airports[i];
    }
    throw exception();
}

vector<string> Manager::split(const string &s, vector<string> &elems) {
    stringstream ss(s);
    string item;
    while(getline(ss, item, ',')) {
        elems.push_back(item);
    }
    return elems;
}

It is throwing this error:

Manager.obj : error LNK2001: unresolved external symbol "private: static struct Vector Manager::airports" (?airports@Manager@@0U?$Vector@UAirport@@@@A)

Manager.obj : error LNK2001: unresolved external symbol "private: static struct Vector Manager::flights" (?flights@Manager@@0U?$Vector@UFlight@@@@A)

I realise i need to define the vectors, but how and where? I tried creating an empty constructor and then doing

Manager::Manager ()
{
    vector<string> flights;
    vector<string> airports;
}

But it just gave me a re-definition error.

Answer 1

You have to define them in the .cpp file:

vector<string> Manager::flights;
vector<string> Manager::airports;

Answer 2

In your .cpp file, you need to add the definitions of the static variables:

vector<Airport> Manager::airports;
vector<Flight> Manager::flights;

See Why are classes with static data members getting linker errors? from the C++ FAQ.