LNK2001 Error with static attributes and methods (Qt, C++)

I have a class EventType, which has the following header (relevant lines only):

#include<string>
#include<unordered_set>
#include<iostream>

class EventType
{
public:
    static EventType* getByName(std::string name);

    static EventType* getByID(std::string id);

    static void setAllEventTypes(std::unordered_set<EventType*> events);
    //...

private:
    static std::unordered_set<EventType*> allEvents; //stores all events
    std::string name;
    //...
    std::string akaID;
};

And the source:

EventType* EventType::getByName(std::string name) {
    foreach(EventType * event, EventType::allEvents) {
        if(event->name == name) {
             return event;
        }
    }
    std::cout << "Error: Event with name " << name << "could not be found.\n";
}

EventType* EventType::getByID(std::string id) {
    foreach(EventType * event, EventType::allEvents) {
        if(event->akaID == id) {
            return event;
        }
    }
    std::cout << "Error: Event with aka.ID " << id << "could not be found.\n";
}

void EventType::setAllEventTypes(std::unordered_set<EventType*> events) {
    EventType::allEvents = events;
}

Now I'm getting the LNK2001-error:

eventtype.obj : error LNK2001: unresolved external symbol ""private: static class std::unordered_set<class EventType *,struct std::hash<class EventType *>,struct std::equal_to<class EventType *>,class std::allocator<class EventType *> > EventType::allEvents" (?allEvents@EventType@@0V?$unordered_set@PEAVEventType@@U?$hash@PEAVEventType@@@std@@U?$equal_to@PEAVEventType@@@3@V?$allocator@PEAVEventType@@@3@@std@@A)".

I get this error even when I'm not using any of the static methods from outside my EventType-class. Why does this happen? Shouldn't EventType be able to link to itself?

You declared allEvents but did not define it, you need to do that in your source file:

std::unordered_set<EventType*> EventType::allEvents;